Yep it keeps the profiles in sync, but see little point in migrating everyone off Plex when they're used to it. Corresponding profiles exist on jellyfin for them if the shit hits the fan. Oh right, well I use Symfonium but I think it's android only.

I'd also recommend importing TRaSH profiles into your *arr stack to make sure you get consistent quality downloads.

Yep, I have an OG lifetime Plex pass but moved over and shifted a bunch of users, but now run both in tandem with watchstate sync in case Plex becomes extinct. Navidrome as a standalone is better than using jellyfin for music.

I run both plex and jellyfin for ~30 users. There's a ton of room for expansion here. Off the Rip, don't use plex. It was worth it when you could get a lifetime license for $80, now, I'd just use jellyfin, it's just the UI and auth you pay for.

You need to make sure you have hardware transcoding enabled for when tour client is unable to play natively.
If using torrents it's important you have it routed through a VPN container or use Usenet, which is much faster and generally has less low quality content.
Moonfin is definitely my favorite client for the TV and Streamyfin for mobile.