Sadly the instructions on the community docs are rather outdated…
I’m always skeptical of free VPNs, but that’s a discussion for a forum like https://forum.privacytools.io.
Given your VPN provider does not give you a .ovpn file that is necessary for installing it with the qubes-tunnel, you’ll have to install it via the .deb as you mentioned.
Installing a untrusted .deb can compromise your template. My suggestion would be for you to have a separate template or a StandaloneVM for making your “sys-vpn” for that exact reason.
I’ve made some instructions on installing stuff on Qubes from .deb files on StandaloneVMs (link bellow). The only difference would be that you’ll also need to tick the “provides network” when creating the qube.
Hope this helps. Good luck!