[3np]

If you have Plex running on that router, yes. Same for Jellyfin (which I recommend) and Emby - as long as you have a corresponding app/client, DLNA, or a web client. If you connect with several clients you can "cast"/remote between them seamlessly so you can play to your TV from your phone, for example.

The library is centralized, though, so you need all the medial files on one place. Also, I'd recommend not running it on your router but on a separate device for various reasons (security, performance especially if you need to transcode). If you want to stream 4K without having to think about codecs you'll need decent CPU and GPU. For 1080p you should be OK with an integrated AMD or Intel, or even a decent SBC.

[silicon2401]

Thanks very much. I've been slowly trying to learn about NAS/home media stuff as I've gotten more into watching TV shows and movies, and your comment is making me lean towards plex/jellyfin/emby on a NAS to centrally store my media and stream it to devices.