I personally use mullvad for all outgoing traffic and then airvpn for any let forwarding I require. Basically airvpn is exclusive to incoming traffic, like my self hosted services or game servers, and then anything I do on the internet routes through mullvad. All setup through opnsense since they both support wireguard.

I always had issues with proton's port forwarding being reliable in the past. That being said, if you need things like video streaming services, mullvad seems to be having a hard time with these recently where as proton worked well for me back when I used it (unsure if that's still true).