I used this: https://github.com/arabcoders/watchstate
It works, really, really well. You just connect it to the servers, and it syncs them by user. You can even let it run regularly while you are in your transition phase as a docker container right next to Plex and Jellyfin.
Mine's even a bit more advanced, as I used samba-domain
to set up an LDAP active directory for my fam, then the above to sync the Plex users to those users in Jellyfin, and it still worked great.
Edit: The WebUI is also pretty intuitive, but I did have to run it twice for my user the first time for it to get 100% in sync. Everything was fine after that.