I'm still struggling with this - presets vs. Composer's prompts.
Think about Counterspell and Temporal Shunt. Think about Silvery Barbs and Cutting Words.
Silvery Barbs. You wait until an enemy succeeds with their die roll. Then the player announces they are casting Silvery Barbs. It could be ANY d20 die roll. How would that translate to a prompt? Every time an enemy succeeds on a d20 roll the game will ask you if you want to use Silver Barbs to make them reroll and then apply advantage on an ally?
And Counterspell. Again. If you are in a battle with 3 or 4 mage's, you could be pausing combat for each and every mage asking if you want to cast Counterspell. And like the duergar Nere fight in Grymforge, they can cast Enlarge, they cast some cleric spells too, and even dominate spells. That would be a lot of prompts until you finally decide to accept 1 - IF you decide to waste a 3rd level spell slot on it.
Cutting Words is like Silvery Barbs. It can happen at any time. I don't know how you'd pull these off without some sort of preset triggers like Selected Roll Types, Selected Spells, Selected Targets, etc. Otherwise, the game would have to pause a lot to ask if you want to use the Reaction.