Keep in mind that you don't necessarily need to "migrate" all at once. There's nothing wrong with running both Plex and Jellyfin pointed at the same media library, and using whichever one has the better client for a particular platform.
I tried this but using two servers means you lose or fragment all your watch history. If I have to remember where I am in a series myself, I might as well just use VLC.
There is JellyPlexWatch, which will sync the databases between jellyfin and Plex. I've been using both next to each other for a while with that setup to sync watch history. Honestly I haven't been able to move over to Jellyfin after the drama around the integration of skip intro, but I hope things get better.
This isn't that important a feature most of the time, because you can just press the skip-ahead button and skip the next 30 seconds. You might see some of the intro, but big deal
There is one big exception though: if you're watching Star Trek: Enterprise. The "skip intro" feature is invaluable so you don't have to be subjected to that truly horrific opening song.
