A "Mini Pro" is what I've been wanting from Apple forever (in tech years). Just a small tower with room for a graphics card and two or three SSD/NVMe drives. I'd be fine with paying Apple's customary 100+% hardware premium for something like that.

That's exactly what I want to.

Is there a Mini Pc that would be similar to this now?

You can do all that now with the mini, it has 4 thunderbolt ports.

I thought it was pretty clear I was talking about something much different from hanging multiple external devices off the back of a Mini.

Thunderbolt isn't nowhere near close in performance to built-in PCI express slots.

I guess it depends on the width you need. Indeed you won't be able to run x16 or x8 but x4 is possible which is enough for full-speed nvme or dual 10GbE