Disclaimer: Plexamp used to be great, but it’s stagnated badly. It was a good reason to buy plex pass at one point, though I don’t think it’s worth it now.
I’m not familiar with Symfonium, but the major defining thing with plexamp is the DJ features for exploring your local music library.
Unfortunately, some months back Tidal support was removed from Plexamp and that was kind of a deal breaker because now it’s only local library, and its “killer app” feature was using the DJ mixes in conjunction with Tidal to do real time mixes with your local and streaming music together.
I’ve switched to using Lyrion instead, along with the Blissmix and “Don’t Stop the Music” plugins with LastFM support. It integrates with Tidal, Deezer, or Qobuz (and I think Spotify, but not sure, I only use hifi streaming services). They work similarly, and in some ways better because you have full control over Blissmix’s functionality for chroma, timbre, tempo, album and track repeats, and more. Also, Lyrion can stream directly over DLNA to a client, whereas Plexamp was just Airplay/Bluetooth/Google Cast (I have Apple stuff, but Airplay is terrible quality).
It’s sad, but plexamp is just my “local download” player now on my phone for when I’m driving, since it downconverts flac to Opus at higher quality than MP3 and at smaller sizes.
I highly recommend trying out Lyrion. I’ve used nearly everything for music in the past, including even having a year of Roon, but Lyrion has replaced pretty much everything.
See my previous comment, I’m 100% on board with Lyrion server and client these days.
I only use Plexamp now for local music when I’m driving.