I have done basically the same thing. With a few differences.
I setup Transmission and SaBnzbd on the NAS but offloaded sonarr/radar/prowlarr + jellyfin to my server so it’s not taking up resources from the NAS.
Found Usenet to download significantly faster overall and sonarr/radar get releases from them much quicker then torrents. Only about 1/15 downloads end up being torrents.
Also overseerr is an amazing tool that you should add into your system. I use it for myself but I also have made accounts for my family so instead of asking me to download something they can just press two buttons and it automatically does the rest.
Agree with the Synology recommendation for a simple starter. Though personally always recommend the 4 bay.