One of the main issues with that balancing act usually is the clear opposition between different types of content. If you want to take into account casual content (including extremes but those sit awkwardly in between I guess), then yes, obviously, if your ranged jobs pull out the same damage output you'll see melees and casters struggle in... casual content. On the other hand, those restraints that melees have to deal with in challenging content have been a non issue for ages, for all the reasons already mentioned all across this thread: boss mechanics designed around melee uptime, melees having enough disconnection tools to deal with the few remaining gaps, etc. Even at lower "percentiles" melees are still ahead in raiding content, which is pretty telling, and I don't remember that the gap lessens a lot as well, because guess what, even ranged players at lower skill levels also drop in efficiency.

Now then the obvious answer has always been, give something else to rPhys to chew on. You removed all of their identity (manasong, raid mitigation) but the heavy raid buffs (except on mch because who cares about mch those days anyway amaright?), then give them something back that is mandatory to take into parties (like old manasong) where it will not be really mandatory at casual level but then the damage gap will do the balancing job there, OR give them more challenging gameplay, whether it's rotational complexity (while staying intuitive, but hard to maximize) or stuff like old cast times I guess (me no likey, that would just turn them into casters). If those solutions are out of the order, then I'm sorry but I'd rather have a balanced endgame savaged experience than a storymode/casual one, where this will get drowned wildly into non existing raid checks, differing gear levels and player skills anyway.