Keep in mind that Counterspell is probably the worst, and I'm still trying to think how to implement it without constant popups if players go up against multiple spellcasters later in the game.
For the rest of the classes I worked on, honestly, there wouldn't be that much Reaction Management involved. I made notes. See for yourself if you want.
Note: It's a Google Sheets doc, so it's not exactly pretty. I hope you will look at it and get the overall idea as opposed to getting hung up on how it looks.
Also, I'm open to suggestions on how to tweak presets, etc.
I think having configurable presets would work extremely well, as long as the defined presets are well defined and correct for each spell/ability type. I see nothing wrong with this at all.
Keeps the combat and gameplay flowing while still giving players the ability to control their Reactions on their turn.
The two I still am not totally happy with are Counterspell and Temporal Shunt. They are the hardest because of just how complicated they are. However, if you have these two, and you have prompts, that would be a nightmare if you go up against a room of spellcasters. Every few seconds, "Use Counterspell? Goblin Booyahg is casting Hex."
I am still pondering them, but a default spell list seems the best way to go. You decide what spells you want to trigger them.
I guess I take a controversial stance on Reactions, in that the way WotC implemented 5th Ed could be slightly changed because it allows too much min/maxing of the type of action.
Instead, I like the idea how you force PCs to make their Reactions either react to things affecting themselves (focus on self), react to things on one specific NPC (focus on target), or react to one specific event (focus on one event).
Allowing Reactions to be omnipotent, the players get to see everything up front before making the reaction, making them feel too gamey. But, if your Reaction has to target one area, either a target NPC, or target event, or yourself, what you have done is made the Reactions seem more as a bonus action to be used to increase success of one's specified action, instead of something that is held onto forever until the perfect moment.
Counterspell is a good example of this as the player gets to see every spell and make a decision after each time. Instead, what your system proposes is the player instead chooses either the first time a specific spell is cast by any NPC or first time a specific NPC casts the spell, Counterspell is now regulated to be a reaction to a specific event instead of getting to see all events all the time then make the reaction.
Same for Temporal Shunt, instead of getting to see every action to make the reaction, you have to either set the first time this amount of damage could happen by any NPC (or first time any NPC uses a specific spell) or the first time a specific NPC could cause this much damage (or first time this specific NPC uses a specific spell).
Bardic Inspiration also gains a lot from this. Use Bardic Inspiration if any attack event could be succeeded by at least +X (since it adds d6 to the roll and lets say you just rolled something that you needed a 6 to succeed, you won't use it, instead you want to use it only if a 5 or 6 succeeds, or within +5 of the d20 roll). These events can then just be applied, just like above, to either the first event or a specific target NPC. Then, like how Larian has the setup, each Bardic Inspiration Reaction version (defensive, offensive, or ability roll) can be customized in the exact same way.