I run JF not plex, but the #1 missing feature I'd like is per-user granular sharing of content within specific libraries.
So far as I can tell, the only way around this is to symlink the subset of media into separate folders, make each of them libraries, then present those libraries to each user.
That's exactly the problem that I tried to solve with the OS I'm working on. You can have a shared jellyfin instance if you want everyone to access the same library. But you can also create OS users, with their own jellyfin instance, and within that you can set access or only to certain folders. That only have the contents that you want them to access.
It's not as efficient as granting access within jellyfin, but it's pretty close to what you're trying to do.
I'm not sure if this is what you want but you can set two lists of tags for each user that works as items blacklists and whitelists. Then you tag whatever media you want with those and they will appear or disappear for that user.
You can find this feature in the admin control panel -> Users -> select an user -> Parental control (tab)
There is also an Access tab, together both can be used to lock things down fairly well.