I haven’t used this in many years so I can’t vouch for the current state, but I’ve used synergy to share the same mouse and keyboard across multiple computers. I think this should enable your 4 monitor dream - 2 externals and two laptops.
You’ll also likely run into a situation where a single laptop can’t drive 4 monitors - keep that in mind.
EDIT: I don’t know why you’re targeting 3 laptops in the future, but I would heavily suggest looking at framework laptops. They’re upgradeable, modular laptops where you can buy new motherboards to upgrade. Once you do upgrade, you can actually use the old Mobo as a desktop machine.
The website suggests it does. I just remember it served me well when I needed it.