Idk what drm message you're talking about, but it sounds like another reason not to use plex.
I recommend jellyfin or emby instead.
You need to set up a dynamic seedbox ip
I have this exact setup. You'll need to setup a bash script to update your client ip, since gluetun will periodically change servers
There are a couple forum posts walking through it, it's pretty simple
.... yea, I agree.
If you're at-all concerned with DRM protections, it's probably better to use a service that is not reliant on making calls to an outside server. Thats not a comment on the factual accuracy of OP's experience, that's just common sense.