Originally Posted by GM4Him
The present preset suggestion is something more like this:

Toggle Reactions Prompts - On/ Off

Attack of Opportunity - All Enemies/ Selected Enemies/ Off

Shield Spell - All Hits/ Magic Missile Only/ Other Only (And it only triggers if the attacker hits by AC +5 or less so that you only use Shield if it will make the enemy miss.}/ Off

Counterspell - All Spells/ Default Spell List Only/ Selected Spell List Only/ Selected Enemies/ Off
Correction. YOUR preset suggestion is like that. I don't oppose your preset suggestion as one of possible options, as long as a more-basic "prompt me" is another option. And honestly I'd prefer entirely reworked reactions (e.g., Cutting Words) over having such detailed presets.

Others' suggestions are closer to exactly what @lilaque said. Every reaction has 3 simple settings: prompt for use / automatic use / never use.
I would suggest a 3-toggle state instead of having to remove the icon from the hotbar. Solidly colored = automatic; blinking or circularly-moving-light = prompt; dim = off.