If you don't need to transcode (on a local network you should never need to) then I am totally happy with my Synology NAS. I have the DS1815+ which is the slightly older/larger version of the + models. I'd assume the newer ones can only improve on what I have. I put an extra 4GB stick of RAM into it though to make it 6GB. I am able to run Radarr, Sonarr, Sabnzbd, and Hydra on this using Docker containers. Extraction of files is totally fine I don't notice it running slowly. Plays 4K HDR files no problem via my Nvidia Shield TV running the Plex client. Only downside is that the Plex Android app is reasonably broken at the moment and has an annoying buy they don't seem to be willing or able to to fix where fairly often the first time I open a file it simply closes and I have to open it again.