A nas may be overkill for what you want.

You may only need a modernish pc with lots and lots of storage so you can keep redundant copies of your library

Plex was super easy for me to set up on my old gaming pc that had multiple TB of storage.

So was jellyfin when i decided to swap.

If you only want to watch your library over your local network and dont care about remote access id reccomend going with jellyfin instead of plex because FOSS vs proprietary software. They both can be configured for remote access but it seemed easier to do in plex and also not something youd want to do depending on where/how you’ve acquired your digital library