Hi mates!
I followed the guide and now I have 256gb eMMC :)
I used a Samsung one (KLMEG8UCTA-B041) and as seen it works perfectly!
I also made some more modules using Sandisk "SDINBDA4-128G" and Kioxia "THGBMHT0C8LBAIG".
Maybe I can provide that service, at least, to Spain (I need to see...