Maybe self host your own VPN on a VPS and connect the jellyfin server as a client as well as any other devices you want to see that jellyfin server as other clients and configure the VPN server to not override your default routing and to allow clients to see each other? In my head I don't think that would conflict with your protonVPN connection.
Your traffic would be encrypted between devices so I wouldn't say https is nessesary and thus no certs needed.
The rubs that occur to me are that I'm not sure you can do this on a free tier VPS which is the only option I see given your financial limitations. And your devices all need to be able to connect to said VPN.
Edit: Slightly less worse English.
Clickbait title is unhelpful but did it's job in making me curious. Guessing it's this for those that don't want to watch a video.
https://www.dexerto.com/gaming/european-union-makes-legislative-change-that-will-change-in-game-purchases-forever-3167394/