Indeed this would be an amazing solution if it worked.
It was actually suggested in Putting DHCP default gateway in different routing table - #8 by acosgrove . Since then I spent countless hours to get it working with no clear resolution (Simple source routing not so simple? (How to migrate this simple RouterOS example to VyOS) - #3 by exp, How can I make my wireguard tunnel accessible via policy routing? - #2 by 16again, Why do my outgoing TCP connections fail when ICMP and incoming connections are OK? - #11 by n.fort).
Either I misunderstand something completely about VRFs or there are big bad bugs in it (I’m afraid it’s the latter). So I am trying to scope if there are any other ways to implement source routing in a multi-homed setup.