The SSDs in these machines consist of NAND chips soldered directly to the motherboard; the controller aspect is handled within the main M* SOC. As such there's no way that a company could offer an internal 'upgrade' or expansion to these drives.

This is not true . The SSDs on this are not soldiered but are in distinct chips that can be removed but none of them have a discrete controller on them instead the controller is on the SoC itself. Basically they are NAND modules not full SSDs at all.

The NAND chips are soldered with ball grid arrays, requiring a reflow oven to add a new chip, not to mention how the board itself would probably not even detect the extra chips.

Here's an M2 Air logic board with 1 of 2 NAND chips soldered on (in yellow), there's an empty spot on the board for another chip: https://guide-images.cdn.ifixit.com/igi/1A2NHYEY6dJcUcd4.hug...

Challenge accepted. Here’s an M1 being upgraded with both RAM and Storage by some Chinese Techs. Yes, you need an oven.

https://www.macrumors.com/2021/04/06/m1-mac-ram-and-ssd-upgr...

I think I was getting the config of the Mac Studio confused with the MBP.

I thought these are soldered together as well? Am I mistaken?

Afaik the storage devices are directly soldered to the 'logic' board and are unable to be replaced by the end user.