Linux supports any VPN protocol pretty much. If your VPN vendor doesn’t want you to lock in their software - they probably allow you to download pure OpenVPN or WireGuard config file that can be easily inserted on any major distro.

And also as far as I knke, Proton VPN and Mulvad has their GUI clients for linux

Book link?

Pretty much all beginner friendly distros have this thing (Fedora Debian Ubuntu Mint). You just have to enable it. Also make sure if you are using secure boot - remove Microsoft keys and generate your own. Also its nice to have bios password setup too.