Complete Beginner realized the need of a NAS?

Hello there!

I'm completely new to the world of NAS-related things and I am overwhelmed with all the things that are possible and equipment that is out there.

What I basically want is a space, connected to our home-network, where my partner and I can move files (Movies, Music, Photos) that is not needed daily on our PCs. As a added bonus, it would be great if I could use the NAS together with either my smart TV or my PS4 to watch movies/listening to music to.
I plan to have 2 4-6TB HDs in there, one for actual storage, one for mirroring the first one and an external USB HD for backing up files externally.

I was eyeing the DS220+ but now that I know of the existence of the DS220j, I wonder if that would be enough for my use case, even though I read on the website that "Not suitable for Plex Media Server". It would certainly be a cheaper solution.
What exactly does the "not suitable" mean? Is it enough for FullHD? Will it struggle at 4k? Does it not work at all? And do I even need Plex Media Server to connect it to my PS4?

I hope you can give me some good pointers to consider, which one I should get (or even a completely different NAS, which I haven't considered yet).

Thank you for your time!

It is not that J series model would not work, but it would be so slow that you would hate it. If you do have a limit on your budget the getting a play series model would be a better choice. You do not need a Plex to be able to access multimedia on your TV or PlayStation. You can activate DLNA multimedia discovery service or use Synology internal Video Station/ DS Video app. Synology plus series model like DS220+ allow RAM upgrade making this a more future proof choice. It also allows more apps to be installed compared to budget series NAS.But all models will offer top quality apps for backing up mobile devices and computers.I hope this helps.