I tried Pathfinder 2 as a caster. Nearly every spell took 2 actions to cast, and just that was a drag. It needed more variety there. I did like the spells that did more based on how many actions you spent casting them, it was a fun tradeoff to make. But there weren't very many of those.Incidentally pf2e has some spells like this. I think they all max out at 2 rounds, but there might be some decent existing templates to borrow.
Total aside, but Castle Falkenstein handled something like this in an engaging way. You had to play cards into a pool each turn to build up your spell power, either to a certain threshold or just for more effect, maybe both, I don't remember exactly. But you still felt like you were doing something on your turn even if your spell wasn't actually going off yet.