Any media manager for YouTube series?

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It's not exactly what you're looking for, but ytdl-sub is what I use and you can manage everything via YAML files and write your own metadata, take it from YouTube, treat it as a series, etc...

I'm not aware of a good metadata manager for YouTube and I know there's a plugin for Jellyfin, but last I checked it only worked for the Movies library type, which is a mess. 

Using ytdl-sub, I create a library for my YouTube series (or add them into my regular TV library/ies) and you're off an running. You control what metadata exists for them. Just make sure to lock it down (i.e., don't refresh the metadata for these series as you might match it with something completely incorrect).