You can just copy over the Library folder, that should do it. Then set up the plex Docker to use bind mounts instead of volumes and place your old library into that bind mount.

I usually do ~/docker/plex/Library and have the Docker compose file in the ~/docker/plex folder

I think this should work? I used this method when migrating my plex server to a lxc container.

I accomplished this and found it was pretty easy. Like everyone else said, just point the volumes to the right directories. Backup your Plex data (not the media) a few times in case you screw up and it starts to index existing files as if it were old files.

Probably the hardest part for me was 'claiming' the server using their startup token. And even then that was fairly easy.

You're hosting on Ubuntu so this isn't a problem, but PMS basically requires its own IP on your intranet (not the Docker internal subnet) so Mac's Docker implementation fails to provide this capability.

If you use hardware transcoding or similar features, you'll have to find out how to pass the necessary devices through into the docker container!

This tutorial does this (if I read the compose fole right). https://drfrankenstein.co.uk/2021/12/06/plex-in-docker-on-a-synology-nas-hardware-transcoding/

Not sure if it is any good, I haven't tried it.