Fun fact: currently it's quicker and easier to manage six characters in Kingmaker than doing the same with even just two or three in BG3. Point and click, drag to select and controlling multiple characters at the same time go a long way to make these tasks effortless. And I should probably stress that I play KIngmaker exclusively with turn-based combat, which means that NO, I'm not talking about "slow turns".
So maybe let's address the shitty control scheme of the game, rather than ask to cripple the side of party management/composition to make thing simpler and duller.
Also, Larian has yet to confirm that they will allow "swapping characters in and out of the party". In the end I expect that they will, but so far the last official word on the topic suggested the contrary ("Player will have to commit to their choice after Act 1") which would make a limit of 4 characters even more shitty. Imagine playing a long-ass single player campaign ranging anything between 60-100 hours and 3 companions would be the only degree of variety offered to you outside of lame custom "mercenaries".