So spell caster that multiclassed in Fighter and with Action Surge can have one more cantrip because of rule In Players handbook, page 202, passage Casting time - Bonus Action (because turn its not over and you already casted spell level 1 or higher), but with Haste no. With Haste action, He can hit enemy with staff or poke him with wand.
To be clear:
"Meteor Swarm-> (timely Counterspell)-> Action Surge-> Disintegrate" Is perfectly legitimate in the official PHB rules. "Shield of Faith -> (throw away dodge action)-> Action Surge -> Magic Missile" is NOT allowed or legitimate, officially. Just to emphasise how stupid the official rule is.
The bonus action rule is, as one might understand from the heading, related only to bonus actions. You only get curtailed on your casting options if you cast a bonus action spell.
The correct answer to this could be allowing PC's to delay their turn to go in the same initiative pocket with another PC.
I think being able to cast two spells per turn is a balance problem. Misty Step + Thunderwave before the target gets any chance to react is already too much. Enemies need to be able to do the same and then it just gets ridiculous.
There are major issues with delaying your turn, and very good reasons why that's not a thing you can do in 5e. Instead, as a means of allowing people to combo or delay for particular triggers and moments, without running into those breaks and problems, 5e has the Ready action.... BG3 does not have the Ready action, however, and that's a problem that should be fixed, but the implementation of the actual mechanics, not by the addition of more homebrew.
Misty Step + Thuderwave is not in any way a problem for balance. Someone under the effects of Fly, or anyone with a natural flying speed, can do the same, and then some, and there are many other ways besides to arrange the same or similar effects.
