https://www.google.com/search?hl=en&as_q=best+nas+for+Plex finds the prior discussions and the recurring complaints are speed. Try to get one that is not WiFi. I know you're tempted but wire it to your network and save the WiFi for other use.

As to sync, I use a few sync apps like Goldsync, Syncback and Roadkil's Unstoppable Copier with great results. So good that I can't pick which I like over the other.

-> "store images"??? Danger signs here. All attempts to have a central storage often end in loss. Not one of these systems are great "storage" as one wrong move, theft, virus or such can replace what we don't backup.

Yes I keep a copy in the NAS drive on our router. The router has a NAS feature so that's what I use. It was cheap too.
Bob