Ya I'm surprised that people are advocating for Plex these days especially in a self hosting community, it's overbloated and mostly exists to force their FAST service down your throat
Plex has to be one thing where I like the bloat. It makes it feel like a fully featured streaming service that I control instead of just a fancy looking video folder.
I'm assuming cause it works good enough for most that the incentive to change it nearly none. Kinda the same deal with moving from reddit, works good for the use case that open stuff might be worse.
Currently using symfonium, does everything I need at least. Can't speak for plex since it been years but it has most of the things you need, offline playback, playlist downloading and syncing, casting, supports transcoding etc. Plus it works with plex already so you can try it out (it's free for a week then paid iirc but it's well worth imho)
I use symfonium too. Great app. I've stumbled across some bugs here or there but that might be due to my setup and if I actually bothered to report them, I imagine the developer would be quick to fix them. At least he was fast to reply the last time I opened a ticket.
One great thing I like about Symfonium is the aggregation of multiple libraries. You can add a plex, jellyfin or subsonic API compatible server (like navidrome) simultaneously.
The player I use for it is symfonium and it can do instant mixes. Genre tagged based tho, so it can be a little wonky but it gets the job done for the most part