In both the Pathfinder games when you are in one of your "hubs" you have permanent access to the inventory of all the recruited companions, active in your party or not.
They also usually mind their own business and you are asked to confirm the party formation only when you are leaving the area.
Simple. effective and it doesn't even require any particularly elaborate custom UI specifically for that goal.
Trust Larian for making simple things unnecessarily impractical/convoluted when it comes to UI, controls, etc.
Yes, I’m playing Kingmaker for the first time at the moment and agree that the way it makes all characters available to level up and tinker with their equipment while you’re at a hub is something I really appreciate. It would be great if it included the items stashed in your chest as well, but that’s not too hard to manage with the party inventory model.