[bibaheu]

>switching ISPs may throw your internal network addressing in disarray which can be either a pain or a massive problem

Those are what ULA's do. A local router that provides global addresses and ULAs solves all your problems, and that's the default behaviour of OpenWRT (and probably other routers). If you want traffic to not leave your local network, listen to your ULA (fd00::whatever) and call it a day.