Not to distract from this but if you're primarily using Firefox I really like Container Tabs for this. With the Multi-Account Container Tabs extension, you can set a SOCKS proxy per container, then set certain origins to always open in a given container. You can even use it with Wireproxy to give tabs an individual Wireguard VPN as an exit route, though Firefox can't authenticate a SOCKS proxy so a secure setup may be challenging.

Sidenote: the Firefox VPN integration is lame. For some reason it's only possible to change the destination per container, but seemingly you can't set some containers to be on VPN and some off. I figured the value-add for Firefox VPN (e.g. over just using Mullvad) would literally be deep integration but sadly it's not very impressive. You can do better with Wireproxy.

Hope you like it! I found it a much nicer experience compared to using the Foxyproxy extension or configuring PAC files.