I think spell slots are really neat when you take advantage of them like 3.5/Pathfinder did. Each slot was prepared separately, could be left unprepared and then prepared out of combat to adapt to the day, and had lots of metamagic options to make use of different level slots.
But by the time of 5e, slots don't seem to have any purpose beyond lowering caster flexibility. When we were playing we ruled spontaneous casters could use spell points instead of slots to give them an edge up on the prepared ones.
3.0/3.5/Pathfinder also used Power Points for its psionics instead of slots, and that worked very well. Shame 5e doesn't have psionics.
I didn't want to pick a fight with 5e people, but we found it unsalvagable and moved on. Anything kind of campaign we could run in it would work better in some other system we know.
No one I know has played Pathfinder 2e since the playtest, but the low success rates and weakened spells made it feel like a very slow slapstick comedy. I heard some of that was from math issues that were later fixed.
We are considering trying it again, but I keep on getting caught up on the loss of things I loved in 1e, like constant flight or not having to take actions for your minions.