I would love to use Mullvad, but I need split tunneling on a per-process basis (Windows), since there is the occasional website that hates VPN-based servers. I have a special browser installation I use for such occasions, but few VPN providers offer per-process VPN exceptions. Any chance Mullvad is considering this feature?
Have you considered running a SOCKS proxy outside of Mullvad (ie on a Raspberry Pi or in the cloud?)
You could then use Firefox Multi-Account Containers to bind a container to the SOCKS proxy, and whenever you need to access a site that doesn’t support a VPN you can just open it with in said container.