I also have lifetime plexpass but plex has pissed me off too much with recent changes that make the experience worse. the enshittification is inevitable, so i just cut my losses and am riding the upswing of Jellyfin. It's objectively not as good yet but i have found some temporary workarounds for the biggest omissions.

Its way too hard to make plex work without internet

I paid $100 a handful of years ago, and it felt worth it at the time. Jellyfin has improved tremendously in that time, adding intro/outro skip, a tizen app, better collections support, and is super snappy comparatively. I thought my streaming server was to blame for the long buffer times at the start of media, but clearly not because Jellyfin starts almost as soon as the player loads in. Plex has gotten worse, more invasive, more expensive, and more sluggish since then. I'll never look back to Plex. What value they offer on top of Jellyfin's functionality (which is shrinking year by year) isn't worth the $2 per phone per month for my new users. I made the switch for ~20 family/friends, and other than the name, people only had praises for Jellyfin.

I need to do the same thing, but how did you do it alongside Plex? Meaning, can it just point to the same library and neither Plex nor Jellyfin gets confused (because of one generating files or paths that messes up the other installation)?