hard disk space

It's very effective. Your storage can be as much as you need, and is easy to increase simply by connecting another device with greater capacity. And external SSDs are comfortably speedy.

And as Matt Haveri posted above, you could easily install your OS onto that external drive and use it as your startup, if you like.

You didn't post which model year Mac mini you have, but if you need more info about compatibility I recommend you check out the offerings at OWC/Macsales. They know their stuff. (unaffiliated, satisfied customer)

You can store larger user files (photos, movies, music, Final Cut Projects etc) to APFS-formatted external SSD. Or install the whole macOS system to it (works on Intel Macs but silicon Macs might be more picky).