The fact that you have a "melee slot" and a "bow slot" is quite telling, actually.
It probably didn't even occur to them to allow two melee quick slots. This design choice reveals a preferred playstyle. Versatile melee+ranged is not the only playstyle or build by any margin but this is how Larian thinks everyone "should" play the game.
Hmm, I think the fact that we can freely swap between one melee and one ranged set without using an action, and that characters can choose to switch to throwing weapons without penalty, is already quite powerful enough. The ability to temporarily swap between melee and ranged attacks and vice versa should enemies not be at the preferred distance for a character is something that it is often useful to have in combat and I think that making that always cost an action would be too severe (at least for a first swap), but extending that same approach to freely swap between various melee weapon configurations would be going too far for me and I think it's fair and right that this is handled differently.
I do agree that it shouldn't be as fiddly as now, but that seems largely to do with the fact that the custom slot setup in the game is buggy. If it weren't, then it would be much easier to set up different weapon configurations to swap to, though at the cost of an action per hand changed which I think is fair enough.
