So I‘m a Synology user for years (currently a DS921+ with a DX517 extension) and use it mainly to store movies/shows.

For you here are some things that might be useful to know:

• Consumer NAS are massively underpowered in terms of CPU and RAM. Both is needes if you run a few Docker containers. Especially the transcoding of media files is very CPU intensive.
• using a very small „compute“ node, like an Intel NUC, takes care of this problem. I run all Docker containers on this one, while I use the NAS only as storage.
• Consumer NAS are super easy to setup and also to scale, in case you need more diskspace.
• I was never a big fan of Plex for various reasons. I use Emby and I‘m very happy about it. I also hears many good things about Jellyfin