If you want to configure a mini PC into a NAS and avoid opening ports to the public internet, one way would be to setup a samba server and Tailscale.
I setup something similar on mine, a “NAS-like” server which is only accessible via Tailscale, and wrote a docker script to make setup somewhat easier (Still requires a little bit of technical knowledge though)
Feel free to read it, use it, tweak it, etc!
github.com/danieldaquino/simple-self-hosted-file-cloud