I think personally smn should be reworked so you build to demis not from them. Instants should become casted GCD'd with the exception of Garuda and Maybe Phoenix.

Beyond that, add something that needs managing / choice. An easy route would be to add three more Arcanum summons, but retain the ability to only use three. Split them into high damage / low movement and High Movement / Utility and provide the choice of when you use them.
Im not good enough to figure out what this would look like on an ability to ability level.

I also feel like demis and arcanums shouldnt remove carbuncle. There is zero reason for it, but make carbuncle function like DRK's living shadow. It runs through a steady rotation, (and give it glamours beyond carbuncles for flavour) Give summoner ability OGCD's that empower or draw out speific things from that constand pet. Hell even make it so cabuncle acts like old Bahamut demi where it will execute an attack based on when you do.

final change id make is to allow banking of arcanum on a minute cooldown for Garuda, ifrit, Phoenix and Temu Banamut. So you can choose when to use it rather than force it within the window it currently is.

Overall it wouldnt likely change much, but breaking the punishing conveyer is needed and would mke the job a bit more difficult.

SMN could be a bit like blm in that conceptually its rotation isnt hard, but executionally is it.

Currently both concept and execution are easy unless you have high ping.