Polysoft, et al. did the modeling science and hand-build units, contracted with Chinese manufacturers to mass-manufacture units, and others then copied/incorporated that design science into their own products.
In the end, making such things at-scale in a 1:1 economical, precise manner involves machinery available only to those at the heart of such matters.
The mentioned add-in cards are produced at-scale, which effectively makes them basically no different than regular m.2 units we source from AAPL, Samsung, SKH or Micron.
Would I roll the dice with low production hand made parts from France? I'm not that adventurous (I'm satisfied with the original 2TB storage in my M2 Studio); but others are, and they seem to have found great success
If I was really interested in upgrading, I'd be more inclined to roll with machine-assembly-platform units (whether they were sourced from AAPL, Polysoft, or a direct-from-source Chinese product).
AAPL users had #9, AE, Titan Tech, Zip, etc. available for their II+, IIc and IIe units back in the day.
Starting in 2006, we effectively had a PC-type level of freedom to switch in/out parts. To this day, I can do the same with my MP5,1 (even to the point that the use of OC allows me to take that even further (with no take-down notice/disable-bit)).
Now, were back to where we were in the 80's: the ability to use after-market mods in limited-port designs.
It's my bet that: if AAPL had intentionally desired to restrict mods, they would have not exposed the slots on these units.
[resumes Lurk Mode...]