Completely forgot that localhost is a special case for secure contexts... Yeah that would either just not work at all or require some tomfoolery with dynamic subdomains which I would not be comfortable with. TOTP would be the go-to then, I think. I agree about separate vlan though, I have a dedicated port without internet that can only talk to web ui for this reason.

Well TOTP need proper time sync, and most routers don't have battery in them