I think that the reason that the SSD is soldered in, and not upgradable, is that it's not standard SSD.
From benchmarks I've seen:
Crucial M4 SSD - 400MB/s read, 256MB/s write
Mac Mini 2018 - 2800MB/s read, 2700MB/s write
There's always the four nice fast Thunderbolt 3 ports for external storage.
Biting the hand that feeds IT
