Mac mini with a PCIe slot is no longer mini. It would need a power supply 3-4 times larger, and it would have to dissipate 3-4 times as much heat. (Memory is already upgradable in the mini btw.)
However, the only alternative is to connect an eGPU enclosure, which means a second box on your desktop with its own big power supply
and a second mains connection... and, last time I looked, still came with a laundry list of caveats about software support (although maybe some of those are for laptops/iMacs which have an internal screen...?)
There is also an option (3rd party) that doesn’t use the MPX slots, it uses the SATA connector on the logic board. I haven’t looked into it but it attaches to the frame iirc. 2 x SATA.
Currently, that only comes with 8TB of spinning rust - maybe someone else will do a 'bare' version since it doesn't look like it needs any special Apple tech (...but, for shame, Apple, its a $6k computer and you leave out the HD mounting cage and PCIe power cables!?) However, even if you used it to add a SSD it would still be limited by the SATA interface (not the
worst limitation you could imagine, but its not going to be as fast as a PCIe SSD). Its really a secondary drive to store bulky files (which is good to have) but not really a replacement for the system drive.
There are plenty of full-sized PCIe adapters that can take 1 or more M.2 cards, which would be the obvious alternative to Apple's modules. The question is (a) do they work in the MP at all (maybe questionable with the ones that take more than 1 M.2) and (b) will the T2 let you boot from them?
Certainly, if you're prepared to pay the price (which is a whole other argument), the MP looks gorgeous - the "locked" SSD module seems like the biggest fly in the ointment. The T2 security concept - where only Apple has the master key - is for iPhones, not desktop computers. Fully encrypted hard drives are a double-edged sword and the sort of users the MP is aimed at should be big enough and ugly enough to make that decision for themselves and guard their own master key if necessary.
I'd say that you're not going to leave the Mac Pro on a train... but someone's already started a thread about how to fly with an MP as carry-on luggage.... There's always one (...actually, on the Internet, there's always 10,000).