Call me a cynic, but I don't trust the dev team at this juncture to do what you're describing with any sort of competence, given their attempts to "solve" tank and healer DPS thus far. There's so many variables in this game that led to the cornucopia of fail that is FFXIV healer design that simply tweaking some potencies and maybe removing an OP spell or two and just saying "lul you're offheal now" just...won't work. You'll have scenarios where maybe a potential "off-healer" like SCH requires more effort but is still OP, so they remain meta anyway, because good players dont shun an increase in difficulty if the reward is worth it, or a situation where they're functionally useless and you roll dual WHMs or something for greater margins of error in your raid prog because they're just easier to play.
What healers need in this game are niches, tbh. When everyone is a "tank healer" and a "raid healer" and a "sustain healer" and a "burst healer" and a "mana-efficient healer" then you ultimately just take whatever does the most DPS or boosts the most rDPS. Note that the niches I described are different from "mainheal/offheal" which honestly I'm not even sure I've seen any MMO do anyway. Yes, having two healers competing for the same niche CAN lead to scenarios where one is objectively better than the other, but that in and of itself is solvable with balancing metrics like utility (which in a world where raid encounters were designed a bit better would mean more than just "DPS buffs"), mana economy, and complexity.
There's more than just that at play here (overall ridiculous throughput relative to health bars is something else, but I won't go into that right now), but healers in this game having zero weaknesses or areas where they struggle, at least in terms of "just healing" is a big part of why WHM fell out of favor. When SCH got ETactics and Indom, what was the value of WHM? Cure III? LOL. When the devs created AST, they had to paste on pretty much all of the abilities they gave WHM and SCH, because otherwise the new healer couldn't compete. That's why AST was frankly the biggest disappointment of new class design I have ever seen in my 10 years of playing MMOs, which is a shame because everything OUTSIDE of its lame pasted-on healer mechanics is great IMO.