+1 for reactions using an interrupt mechanic. It is important for so many reaction spells.
As I and others have stated elsewhere, this doesn't actually slow down combat because it gives you things to do in between PC turns. You get to do more things more often.
Larian could combine their automatic reaction toggles with interrupt prompts as @Zzealot suggests. This would prevent prompts from appearing if you don't want them to. I somewhat worry that this would end up being more work, where each turn you might have to turn off/on multiple spells. At that point, just giving us prompts may be faster.
Also, this is a perfect toggle option. People who want automatic reactions? Flip that toggle switch to 'automatic'. People that want detailed control over reactions: flip that toggle to 'manual.'