The problem with modifying reactions to become bonus actions is, of course, that many characters already have bonus actions that they now cannot use. Dual-wielding paladin cannot take an off-hand attack if they used a bonus action to smite.
If you, however, mean that reactions should be FREE actions during the players’s turn, we might get somewhere. Unfortunately several reaction abilities, such as counterspell, really need to be used selectively or they waste resources.
I think using Bonus Action for off-hand attacks was a mistake to begin with. I would separate that first. Bonus Action by definition is a bonus in addition to attacking, no need to nerf dual wielding by effectively removing their bonus action. BG3 makes this even worse because dual wielders now get penalized also for jumping, drinking potions, shoving... list goes on. But that's more of a D&D thing and Larian just made it worse.
Free Action could work better in many cases, yes. The essence of it is making these abilities active instead of reactive to get rid of the problem.
It's just a fact that a 100% accurate reaction system will never be good in a video game. It just destroys the flow of combat by constantly prompting for every little meaningless detail. And the automated system we now have fires off reactions at wrong times, or not at all when you need them to. That's why I would fix this particular issue with the axe.
