Casting more than one leveled spell per turn is already such an obvious balance problem. Of course D&D rules need to be followed here.
We have examples of Sorcerers effortlessly soloing the most difficult encounters in the game without enemies even getting a turn. Double surprise rounds contribute to that as well and need to be fixed.
And spellcasters are still "weak" at these levels. The alpha strikes will get exponentially more powerful at every new spell level.
If Larian insist on keeping this in because "being OP is fun", this nonsense needs to be contained in lower difficulty settings.