A lot of people have suggested Tailscale and it’s basically the perfect solution to all your requirements.
You keep saying you need ProtonVPN which means you can’t use Tailscale, but Tailscale actually supports setting up an exit node which is what you need. Put Protonvpn on the Raspberry Pi, then set it up as an exit node for your tailnet. There’s a lot of people talking about how they did this online. It looks like they even have native support for bypassing the manual setup if you use Mullvad.
As long as every client has the ability to use Tailscale (I.e. no weird TVs or anything) this seems like it checks all your boxes. And since everything is E2EE from Tailscale, TLS is redundant and you can just use HTTP.
Tailscale is just a bunch of extra fancy stuff on top of Wireguard. If you don’t need the fancy stuff, using raw Wireguard can be more lightweight, but might require more networking knowledge.
The biggest thing Tailscale brings you the table is NAT traversal. On top of that it uses direct Wireguard tunnels as necessary instead of creating a mesh like you usually would if you were using raw Wireguard. It also offers convenient bits of sugar like internal DNS, and it handles key exchanges for you so it’s just generally easier to configure. When you do raw Wireguard you’re doing all the config yourself, which could be a pro or a con depending on your needs—and you’ll be editing config files, unlike Tailscale which has a GUI for most things. It also supports some more detailed security options like ACLs and I think SSO, while Wireguard is reliant on your existing firewall for that.
Here’s what Tailscale has to say about it: https://tailscale.com/compare/wireguard
I’ve messed around with Tailscale myself, but ultimately settled on running Wireguard. The reason I do that though is because I trust my LAN, and I only run Wireguard at the edge. Tailscale really wants to be run on every node, which in turn is something that raw Wireguard theoretically can do but would be onerous to maintain. If I didn’t trust my LAN, I’d probably switch to Tailscale.