Next step in data storage

Hi all. I currently have a SFF pc (HP Optiplex-like but smaller) running Plex and some other software. Happy with my software setup for the time being.

However - I'm stuck with what to do next on the storage front. I have a 5TB external HDD that's plugged into the little PC and that's basically what I use for all of my storage. This isn't enough (running out of space) and also doesn't feel very versatile.

Where to from here? I don't know whether to go the RAID route and buy an enclosure that supports it, or just get a bunch of drives and chain them together. A Synology NAS seems way too expensive. What should I do?