Ok, so your path and mine are very similar indeed: I also played 2E for a very long time, than switched to 3.X and stayed there for a long while. I loved it, but eventually the bloat became way too laborious to handle, and fights could drag on for hours. I skipped 4E entirely, then started 5E and really enjoyed it: DMing it is a breeze, fights are way faster, etc.
5e has some aspects I don't like too
- customizability is good but could be improved (I actually detested the stupid number of prestige classes and feats i 3.5). I'm ok with how feats are few but quite powerful in 5e and prefer getting a feat than reaching 20 in an attribute
- there's no real guidance for magic items creation and pricing
- official monsters and some spells are broken against their own guidelines.
- encounter building guidelines are vague and often useless if not harmful
- combat is fast but easily repetitive, especially for martials and most melee monsters
- strongholds are not even mentioned
- very limited weapon properties etc
A5E is a straight up upgrade wrt 5e in all these aspects and many others. There's layers upon layers of greater customizability, combat is way more tactical and interesting, weapons and armors have meaningful differences, monsters are interesting and way more balanced, encounters have quite a more predictable difficulty, etc.
Still, it has to be compatible with 5E, so some aspects couldn't be changed:
- attributes are still capped at 30 no matter what, and normally characters cannot get an attribute above 20, although you can get to 22 if you get the right stronghold (without magic or special class features)
- advantage and disadvantage work the same way. I like this as I hated tracking down every source of bonus in 3.5, but I agree that it's a big sacrifice in granularity. A5E introduces expertise die, which are added on top of a roll, and while they are not as granular as 3.X circumstance modifiers they are a decent compromise IMO. I'd personally suggest not stack sources of advantage and disadvantage as it would create a race to get those additional sources, halting the game to a grind
- bounded accuracy is preserved, so you won't see a character going from a +1 to a +20 modifier in 20 levels. This can be hard to accept at first for a 3.X veteran, but it's actually a good thing. Characters are differentiated by their features, feats, spells and equipments, not by their "+X" to action Y. You don't need to be concerned that some rolls will be utterly impossible for some characters and absolutely trivial for others, albeit there will be situations in which the difference in chances between characters can be very wide.
So, if you're expecting LU to be closer to 3.X than 5E in those fundamental aspects, it's not, and that's due to compatibility issues. If you're mostly ok with 5e but want something more customizable and reliable, then I'd say it's definitely for you.
Regarding WOIN, I absolutely love the system and I think it's definitely worth a shot for every D&D player. It's a very different system with incredible customizability due to its modular design. It may address all your concerns as:
- the classless design means you can mix and match pretty much everything, and purchase individual exploits as you like
- combat is very tactical and positional
- there are several modifiers which do stack, adding or removing to the dice pool you throw
- you can willingly take a penalty on some rolls to get additional benefits should you succeed
- skill and attribute checks are freeform
- the power curve of characters is unbounded, although there's diminishing returns.
There's quite a bit of material for it, thanks to EONS, although no official material was released for almost 2 years. This hopefully will change as the Starter Set was released a few days ago. I really hope for a WOIN resurgence, and I'm eager to give it a try with my group.