This may or may not help, but I went through the exact same sort of struggles that you did when I first started. Setting up all this was my first forray into Linux and learning about permissions and file systems was a hurdle but having conquered that my primary issues were always caused by one of the following:

• file path mapping
• permissions
• port routing in gluetun
• gluetun/mullvad incompatibilities

Now that I actually seem to have a handle on all this…mind you, I’m no expert, but I can walk people through it in plain english. I eventually ended up switching to Proton VPN and use cloudflare tunnels to access my services from outside the network.