That seems pretty easy to design around. You could have powers that ramp up over turns instead of being nothing for several turns and then the big effect.

So like a maelstrom spell. The second round reduces movement speed of enemies, the third round does small damage, the third round immobilizes, the fourth round does big damage.

You can tweak a lot of variables there to make it tactically interesting. How many rounds before the first effect. How good are the intermediate effects. Consequences of being interrupted. Choices to make mid-channel. It could be a very cool way of doing spellcasters and it doesn't need to be per-rest at all.

Spells that need a longer out of combat prep time would also be interesting, like your magical trebuchet.

Honestly if I was going to do a dnd-like game, sorcerers would be like my ramp up and wizards would be like your trebuchet. Wizards would be bad if they were surprised but they could build very specific spells ahead of time.