openvpn-multi-authentication-plugin VS vopono

You should take a look at OpenVPN plugins. Any authentication can be provided via plugins. For example https://github.com/osenchenko/openvpn-multi-authentication-plugin

Vopono (https://github.com/jamesmcm/vopono):

"vopono is a tool to run applications through VPN tunnels via temporary network namespaces. This allows you to run only a handful of applications through different VPNs simultaneously, whilst keeping your main connection as normal.

vopono includes built-in killswitches for both Wireguard and OpenVPN."

I think for now on Linux you (still!) have to use external application to do the splitting. Such is vopono

I use vopono to manage it for me with Mullvad though.

you might want to check out vopono, i've gotten it working with firefox and its nice

https://github.com/jamesmcm/vopono

I highly recommend using something like vopono with this on Linux that runs only the target application (e.g. Transmission) in a network namespace in the VPN and only forwards the specified ports and only allows traffic via the VPN.

This is why I use vopono - you can run just the torrent client in the VPN on its own and the killswitch keeps it safe.

You can find the option in the settings of windows and android apps. Unfortunately, it's Still not available for Linux. As a workaround for Linux, you can use vopono, which has OpenVPN support for Proton. Or just manually add routing table entries :)

I wrote vopono, I use it all the time for quickly spinning up browsers through different VPNs for checking geographical restrictions, etc.