I had a much wordier post before I realized it could be simplified a lot, this is the simplified version.
There are two "phases" to most games - the "between the sessions" phase (usually during level up and/or daily spell preparation) wherein the players are choosing from which among "all available options" they wish to kit their characters with for the next "at the table play" phase, and the "at the table play" phase.
I am of the opinion that the rules manuals are vital in the "between the sessions" phase... and should be forbidden during the "at the table play" phase... because I feel players should be responsible to ensure all relevant rules information in the manuals for the options they have chosen to kit out with should be available at a glance on their character sheets (with possible extensions, for example, I think Spell Cards are a great innovation so a caster carries - depending on level and edition - a half dozen to a couple of dozen spell cards in addition to the character sheet). I have yet to meet a game system where every important bit of information about the options a character has "during play" do not fit on a single sheet of paper (Front and Back) plus Spell Cards for casters.
No matter how "rules heavy" or "rules light" the system, if players feel the need to resort to manuals during "at the table play" in order to select the best option to resolve a challenge, that to me is a failure of the players' preparation, not the system itself (exclusions apply for "newness" - a new player to a system won't know what to prepare yet - but veteran players of a system should know what they will need).
Likewise, it is worth noting that the more "floating modifiers" have to be tracked and aggregated to resolve a situation (e.g., status effects, conditional effects, etc.) the more slowly a system will be to resolve during "at the table play." Here, the design of the rules CAN serve to speed up or slow down the resolution (for example, 5E's "Advantage/Disadvantage" mechanic is much faster to resolve than 3E/PF2E's plethora of +/- bonuses, especially when you have to remember that multiple bonuses of the same type, e.g., status bonuses, don't stack). So this is a case in which "rules light" helps resolution occur "faster" (which may or may not be better).
Let's also note that for some players, the "Resolution" isn't the part of the game that they enjoy, but rather the "Accumulation of Bonuses to increase my chances of Resolution in my favor" - that kind of player tends to prefer "Rules Heavy" systems (more bonuses to try to accrue) and so what passes as "fun" for one type of player may be "boring" for another kind (in either direction, some people love Advantage/Disadvantage for its speed and eschew stringing a dozen bonuses together as tedious because it's complicated, while others love stringing a dozen bonuses together and think Advantage/Disadvantage is tedious because it's simplistic). So what is "most fun" for "at the table play" will vary from person to person. This means asking "is this fun" is flawed - the only really objective metric we can use is "how quickly can things be resolved" (and this is completely orthogonal to fun - DM fiat for resolution is the fastest resolution method, even faster than flipping a coin - but most would not call it the "best" or "most fun").
So, let's also point out that most systems are actually far more rules-heavy in the "Character Kitting" portion of the rules (e.g., Pick a Subclass/Feat/Prepared Spell) than in the "at the table play" portion of the rules, whether they bill themselves as "rules lite" or not.
Finally, let's go back to the idea of Dungeon Exploration. For most people, the idea of "Dungeon Exploration" tends to conjure up "an exercise in resource management" - be it torches, arrows, hit points, food, or what have you. However, there are some in this thread that have posited that that isn't what they see as part of Dungeon Exploration (where to them it may be about the tenor mood evoked by the area as much as a "resource management" style of gameplay). So we can't even all agree on what "Dungeon Exploration" is supposed to be, much less measure whether or not it's supposed to be "fun" (which we also can't agree on).
Thus, this thread has become simultaneously a disagreement over "what constitutes Dungeon Exploration" and "what constitutes fun" - both of which are ill-defined - and to a lesser extend "what constitutes Rules-Light" (again, different people will draw the line at different levels of rules for what they consider "light").
Having said all that, let me try to reframe the original problem. Reynard expressed that PF2E (Abomination Vaults) and 5E Rappan Athuk, as well as "dungeon crawling with PF1 and 3.x era D&D" were all "failures" but I think his phrase "rules made dungeon delving a chore" is instructive here, along with "unwieldy, complex systems are slow and turn the crawl into a grind." Here's what I think the complaint is (and
@Reynard, sorry if I am putting words into your mouth here):
A dungeon delve tends to be monolithic in tone (the area doesn't change, the architecture of the dungeon doesn't usually change, and so generally a "dungeon delve" - which I take to be "clearing a large dungeon" - tends to feature a lot of the same repetitive gameplay (combat) against similar foes in similar terrain over and over again) and Reynard finds the long string of similar encounters in similar terrain tedious - and thus wants a system that is fast so he can get through the "same encounter reskinned multiple times" as quickly as possible and do something new. It's not that he doesn't like "resource management" or that he doesn't like dungeons per se, it's that he craves variety in his games and a large dungeon doesn't provide enough variety.
(Yes, there are contrived dungeons that could do this, but in general, medium to large dungeons are "same" everywhere or close to it, you might have a "finished dungeon" attached to a "cave system" but it's just two big back-to-back "same" areas.)
So
@Reynard, may I humbly suggest the problem is not with whether or not the game system is Rules-Light or not, it's that dungeons by their nature tend to be somewhat monotonous and you would like more variety in your sessions. Personally, I am of the opinion (not fact, my opinion!) that some degree of resource management is desirable in RPGs and that Dungeon Crawling as a style is something that probably does a better job emphasizing the resource management portion of a ruleset more quickly than other styles (since torches burn out in an hour; when hex-crawling, water and food tend to run out on the order of days).
There are some that feel that Resource Management in games is boring. It is my opinion that since games are the method by which we as humans "practice" real life with far lower stakes, and that since real life requires a lot of resource management (time, money, energy, etc.), I prefer games that bring resource management to the fore (it's why I prefer, say, Sorry - in which "pawns at home" is a limited resource since once I bring all my pawns out of "home" the "Sorry" card is a dead draw - to Snakes and Ladders - and while I don't want to use spreadsheets for my fun, I do like to have some mild amount of resource management in most of my games) and in fact think it is healthy for games to require some level of resource management. Gygax's quote "
You can not have a meaningful campaign if strict time records are not kept" is to me less about timekeeping per se and more about the importance of teaching resource management.
What is my point here? I'm not sure, this was mostly an exercise in me reacting to this thread free-form. But I think I have decided that:
1. Dungeon Exploration is a genre of RPG adventure that generally brings resource management to the fore in a more immediate way than other adventures tend to, and does so in a "personal" manner rather than an abstract manner
2. I need to encourage myself and my players to prepare their character sheets better in order to reduce "at the table" use of manuals to speed up play, regardless of system. Maybe as a community we should be demanding character sheets that remove the need to reference manuals at the table... and maybe VTT's "doing the math for us" is one convenience that VTT's provide us that we are overlooking (we tend to focus on the ability to get together with groups remotely and on the visuals VTT's provide, but have we considered that a good VTT tends to be a database that almost instantly pulls in exactly the rules we need and none of the rules we don't)?
3. Rules Light systems tend to let the game resolve faster. That is only "better" if I'm interested in getting the resolution over with faster for some reason external to the rules (for example, this could either because I'm bored with gameplay or uninterested in what is being resolved and want to move on to something else... but it could also be because I'm
extremely invested in the outcome and impatient to see the outcome as soon as possible). Drawn out resolution can be engaging for any number of reasons (suspense, tension, some interested in a high level of detail, etc.)... most inhale food, for example, because they enjoy savoring the smells, texture, taste, or combination thereof; in the same way, my goal is not to "get to the end of a D&D session" but instead to enjoy the journey so "faster" is not always "better" - but there are some sessions that are tedious for whatever reason (as an example utterly unrelated to system, imagine "conflict among players unrelated to the game") and yes, for those sessions "faster" is or course "better."
4. The "perfect system" does not now and never will exist because different people like different levels of complexity in different aspects of the game and by designing to one person's taste, you design against someone else's. The "perfect system" will be perfect only to you.