Expanding storage on simple home server

Hi all, a few months ago I got started with selfhosting. Installed Ubuntu Server on a HP EliteDesk 705 G3 Mini. It's been great, running Jellyfin, Tandoor, Calibre-Web, and Miniflux. Everything is local access only.

The machine came with 1TB SSD and currently about 80% of that is taken. I've been searching around for good options to expand. While I'm relatively comfortable on the software side of things, I'm very inexperienced with and somewhat intimidated by hardware (but would love to learn a bit more).

What would be the most prudent way to expand storage? Is it simply replacing the existing SSD? Should I think of adding a NAS instead?

Buying new hardware would be ok, my only hard requirement is that I don't want to run proprietary software/OS.