Almost any distro will work for pretty much all of that. Only potentially limiting thing I see there I don’t think that the Mullvad VPN client is generally available within distro repositories. Their site says it has clients available for Ubuntu, Debian, and Fedora. So perhaps pick from one of those?
For the steam link question, I know if I have a game running on my desktop it presents me the option to stream to my laptop (both Linux machines). I assume this is the same functionality. I don’t use it so can’t speak to how well it works.
Edit: It occurs to me this comment lacks a recommendation. Within the three listed supported distros for Mullvad, I think I’d recommend Fedora for you. Been some years since I used it last but I remember the installer being fairly robust and intuitive while still easy to use, and a pretty rock solid system once it’s up and running.
My team used to be pretty single threaded – only one who knew how to support a given application/service/whatever. Some of which are critical to have running and are used 24/7. Which made this very thing a real problem.
Things are better now though. We’ve been cross training, so even if the subject matter expert has time off, there is likely someone else who can handle most things.