I think the only thing the combat system needs is a dedicated way to go a single step forward while paused (for when you need to be tactical about bosses), though I also agree with the other suggestion here as the dragging and dropping can sometimes be annoying. A way to quickly move items to and from inventories without dragging would be nice too (such as shift-clicking to immediately sell/buy it or move it to/from the chest, as relevant).
So, funny thing. The game does have a dedicated pause key (space bar by default) but its' behavior is absolutely scuffed. You would think that it would be a press to toggle, but instead it's a hold to temporarily pause the combat, because after a couple seconds it will advance a tick anyway.
HOWEVER, you can use it while manually toggling combat pause with the button on the combat UI to basically achieve an effect similar to manually advancing combat a single tick. But honestly, the pause key should've been a toggle to begin with.