Yes, it only has TB1, but USB3 is pretty much fast enough that an external SSD with USB 3 might -- might -- be only a 10% decrease in throughput compared to an internal SATA SSD
I setup a 16gb base 2012 mini to boot into Sierra from a 500gb USB3 Samsung T3 and a 16gb 2.6ghz quad mini with a 1TB Samsung T3. Performance is basically the same on both disks - 393write/433read on the 1tb T3. My 2012 quad has the original Apple 256gb internal SSD and gets 450write/517read. That's significant, but still feels very fast and beats having to open it up and risk breaking something.