I can't believe I'm actually agreeing with you on this one, and that you addressed the one point I forgot to.
That said, I acknowledge the internal mac mini hard drive has faster sequential read/write then an external SSD, but for most applicable uses (excluding extreme circumstantial pro use) , it's the same difference as tasks are bottlenecked elsewhere and the IOPS specs are comparable.
It doesn't tho, if you get a nVME SSD via thunderbolt3.