I'm late to the thread but understand you haven't started playing 5e yet. It is one of my favourite editions so I'd like to see you try and hear how it goes. For the record, my favourite editions are BECMI, 3.0 and pre-Tasha 5e, very different from each other
Most of my favorite RPG systems over the years are pretty crunchy.
I do not consider 5e very crunchy.
First of all, I find 5e much easier to DM than previous editions, especially if you have a "rules as a toolbox" approach to the game and always keep Rule 0 in mind. If you have a heavy RAW approach to the game and want rules for resolving everything, you are going to encounter troubles when dealing with exploration and stealth, which are purposefully left more open than previous editions.
Also from a player's POV the game is lighter than 3e, but it can still be fairly crunchy if you want. The point is that 5e was designed to accommodate both players who want it to be simple and players who want it to be complex. I can't say it succeeded spectacularly, because IMO there aren't many character options that are truly simple beyond low levels, and at the same time there aren't options as complex as in 3e, but the range is fair and most importantly the game doesn't penalise players who choose simple options.
On a single character, 5e offers less "building blocks" and dials than 3e, but 3e was probably the most extreme editions with regards to this. 5e tends to offer bigger building blocks with more dramatic effect, think Duplo instead of Lego.
Across all characters, 5e offers a much smaller amount of published options than 3e. But because they are bigger options in smaller amount, 5e options actually feel more important.
The irony is, that 3e had smaller building blocks because it was meant to be played by people interested in creating their own material (see DMG notes on Prestige Classes) and ended up selling tons of books with ready-made ones because most players rejected homebrew material in favour of "official" one.
I’m looking to see how 5Ed supports concepts that are distanced from the core D&D PC designs of the past 40+ years.
That is tricky. 5e offers new concepts since the start, both narratively and mechanically, not necessarily entirely new but at least uncommon or previously non-core. For example Totem Barbarians, Paladins of the Ancients and GOO Warlocks felt quite fresh to me (not saying they were truly new to everyone, but I haven't seen them before myself). Wildshape and Combat Superiority mechanics also felt fairly new to me.
On the other hand, I don't think 5e makes it easy for you to create novel material by yourself. 3e seems to be a lot easier to break down to numbers, which is probably boring on one hand but makes it easy to create homebrew stuff balanced with the rest. In fact, even official 5e supplementary character material is few and often mechanically repetitive (bonus damage, advantage and extra proficiencies make up for most special abilities).
After more than 45 years in the hobby, I’ve played most of the stereotypical race/class combos from pre-2Ed that lingered on even afterwards. So when I sit down at a gaming table to start a new D&D campaign, I have those character concepts ready to go and can generally create one pretty quickly- I’ve even played some in 3.5 while I was still generating the character (I was late for the first session). But I’d much rather try something askance from the same old same old.
So being able to create and play things like a full caster in heavy armor, a “Paladin” with arcane spells, or a 2-headed bipedal fey dog-man engages my creativity at a different level. I invest more in the PC.
So 5e offers some of those possibilities already since its core. If you start by choosing a subclass that piques your interests, possibly multiclassing (5e has the best multiclass spellcasters rules if you want) and choosing non-obvious feats, you have the potential to create original characters (I woouldn't give backgrounds much mechanical importance though), but probably can't go very far before you start looking for supplementary books. Even there, I find the amount of available feats and spells still lacking after 10 years.
People just don’t learn like that. It’s divorced from reality in a way that really bugs me. A bridge too far. It pushes me out of my willing suspension of disbelief.*
Such feeling won't be easily overcome. 5e is less concerned with realism than you may like. Certain 5e tenets were chosen for gameplay reasons at the expense of realism, and the proficiencies mechanics is one of them. Another example is the full healing on a long rest. There is no other way around them than to house rule them if you don't like them. But OTOH with 5e being less crunchy, house ruling that kind of thing can be done without fear of having to rebalance other things.