@kwf okay, and since you don't want to be stuck on USB 2.0 speeds, you still need to splice 5V DC from USB.

So unless one were to solder - like you did, only a MiniPCIe to SATA-6G adaptor would've been an option.
https://www.ebay.de/itm/284350479783

The "thiccer" #hp #t620Plus would've had a #PCIe x4 slot and even more space as well as said mSATA slot populated.

If I had a 3D printer I would've likely made a custom lid with space to house more drives and an HBA:
https://www.ebay.de/itm/295105489676