Honestly this is probably a fair question to ask, provided we're not trying to get too into the nitty-gritty details numbers-wise.

I don't think it'd be too hard to make different feeling rotations that engage people in different ways, and even even have different button counts so some immediately appear more or less easy than others. Personally, I think the most obvious way to do this is would be to make them loosely analogous to the available casters, and would probably go a long ways towards easing non-healer mains into the role as well!

So what does that look like?

For starters, WHM is probably going to be taking on our pseudo BLM rotation. Given that we can't use MP as a sliding scale and need to leave room for potential GCD heals, however, I'd approach it from a different direction than the stances. At it's core, we'd mostly be working with 3 DPS buttons: Stone, Aero, and Water. Stone is our filler spelll, and it has very, very low damage and a full 2.5 cast time, making it turret-y. Aero would be a GCD Cooldown ability, probably on a 30/40s recast, and instant cast. It'd be a dot analogue, but without the need to track the dot. To make it a little more interesting and give it some kit cohesion, it'd also decrease your GCD by 20% or so. Water, then, would be the Big Damage, Feels Good To Press button. This would also be a GCD cooldown spell, probably also 30s, but with 2 stacks. Which isn't terribly exciting on it's own So how do we make this more interesting? Add an effect to your filler spells: every time you cast stone, reduce the cooldown of Water by 5s. Anytime you cast a normal GCD heal, reduce the cooldown of Water by 15 or so seconds. Bake in a soft dps refund, and make it so that you're still constantly building up to that one button you want to hit the most. (I've honestly been thinking about how I'd rework WHM a lot the last couple of days and might most a full rework if I get bored at some point...)

SCH would probably be less popular with older healers, but if I was rebuilding it today I'd probably make it look a little more like SMN and make the disconnect between the two less strange. So rather than DoTs, I'd lean into the ability charges you look for opportunities to use sort of thing SMN is attempting to do. You keep aethercharge as a damage GCD ability, but rather than the primal gems that unlock extra attacks, SCH would just directly get a use or two of a couple different spells that they'd be looking to places to use over the next minute before the next aethercharge happens. You could put your mobility ruin 2 type of thing here, maybe a long cast high damage spell, one spell with a very short duration damage down effect, another with a vuln debuff, and then a damage+heal effect, a bigger damage melee spell, a spell that boost a handful of your next filler spells, medium length dot you need to be aware of potential target jumps for, etc. Or whatever else winds up making sense and would make the timing flexibility useful.

SGE is probably were I'd do the RDM style rotation. It probably doesn't something similar to the melee combo, but like, RDM/DNC style procs to build up to a big laser seems like it'd be a great fit for the class. Bonus points if the procs all have different kardia effects, so there might be times you want to hold them a little bit to make better use of their defensive properties rather than just immediately dumping for damage.

That leaves us with AST not having a direct comparison, and coincidentally it's also the one that's thematically the most different lorewise. I'd personally lean into cards being a bigger part of the base rotation and make them happen more frequently, but also on the GCD. And then to augment it, I'd let it have two dots with different durations, to build off the time mage / manipulation themes we've lost over the years, would would also make it feel different from the other three which wouldn't use many if any dots and would wind up using their damage filler spell much less often.

There's probably a ton of other ways you could do it, but IMO that's how I'm feeling at the moment. I'd definitely like to stay away from anything that looks too much like a melee 1-2-3, as well for what it's worth.