Playing an expansion as current content has been a lacklustre MMO experience

[Shurrikhan]

The problem with unlocking abilities in Sync with lower duties is that it'll undervalue any lower-level players whom dont have all of their abilities unlocked in that job. No matter how much you crunch the numbers. The best solution, and I believe seeing how they reworked Monk to give its "lite" Chakra gauge playstyle is to rework the abilities classes unlock to be available much earlier in levels but like you said, be lite versions of those abilities. Examples include:

White Mages having access to lilies
Bards having access to all 3 songs for their rotations
Dragoons having access to their dragon abilities
Gunbreaker having full access to their continuation combo (rather than the gimped one you do pre continuation....ugh...)

etc but basically allow these abilities to be unlocked much, MUCH earlier in the leveling progress. It'll be slightly overwhelming for new players of course but the big plus is that it'll also help to understand their job's playstyle once they reach max level rather than learning and relearning their rotations within certain level ranges

These are all decent compromises, but I have to wonder why you think that "threat" is significant.

It's not as if level capped-players are going to spam-run levelers right out of leveling dungeons. We don't even touch any of the affected content outside of leveling roulette or joining friends to help them or share their player/rl experience as they first-run it.

So what if level 90 players have a larger (number-crunched) kit to run Sastasha with. They're not about to start optimizing speedruns for it, let alone push out from it the players necessary for that or any other leveling dungeon to be of any use to them (by filling the leveling roulette). Unless poor number-crunching would be the unique reason the content becomes trivial when a leveler unfortunately fills their party with 3 level-capped players, it's irrelevant.

That "problem" isn't a problem.