I just got my first Synology (DS220+ 6Gb) a few weeks ago. I went straight to DSM 7 from the start.
I had been using Plex on PC for over a decade. But when I put it on Synology I went for a 100% from scratch installation. No database migration (I wanted it completely fresh). So my database was built from the ground up. My only issue is that the intro scan detection and video preview thumb building hogged the processor darn near completely, almost crippling everything else running. However, after all those scans and building were done (it took me about a week with a 22Tb library for shows and movies) Plex runs without problems.
As far Channels (Synology package, not Docker), it runs like a dream. Detected my HDHR Connect and Prime with no problems. Accepted my TVE with no problems. Accepted custom channels no problem. Multiple client connections no problems. The only time I had a problem with Channels was when Plex was still setting up and hogging the processor.
Currently running ChannelsDVR (2022.05.06.1650), Plex (1.26.1.5772), Docker (20.10.3-1239), Sonarr, Radarr and Bazarr with no issues or conflicts. DSM 7.0.1-42218 (have not upgraded to 7.1). Basically once I got everything setup (after a lengthy initial Plex setup) it has been awesome.