Very interesting combat mechanic, could be a lot of fun but needs a bit of tweaking.
I had a problem where hold clicking on an action only worked about half the time and it was extremely frustrating. I feel like the combat wants the player to react to the enemy but the timing seems too fast for that. It would be nice if a single click moved an action straight to the next available slot. Then hold clicking can move things. It didn’t seem that actions could swap with one another either, they could only move onto emtpy slots. I might suggest trying things like having the enemy actions start populated to allow the player more time, having the enemy take no action every 2/3/4 slots depending on the quickness or difficulty of the enemy, maybe have a pause time mechanic to allow the player more time to choose actions but then actions have to have more weight to them or some type of pros/cons system.