I thought I had seen something about that but couldn't find the actual boards mentioned for sale. Sounds even worse though because it should be possible but Apple being Apple has ensured there's no source for Giles or other companies to perform repairs.

There's an industry in China for desoldering and reusing apple bands. Unfortunately getting new oem nands is going to take legislation.

Yes, though that relies on the product being fairly popular and for the chip to be stable for a while for it to be a useful source. Mac Studio NAND chips aren't going to be readily available from them unless they happen to be a shared part from a more popular device.

All macs use the same compatible nand chips.

Is there any compatibility between generations, eg. harvesting NAND from a M1 generation machine to upgrade a M3 (or now M4) generation machine?

As far as I'm aware, yes they're all compatible.

Apples segmentation takes place in the nand firmware. The firmware contains the location in the storage configuration. And this may or may not be rewriteable. Iboffrc has done a video explaining how some of it works. It's all from reverse engineering though.