Thanks for the replies.
The truth is, I can't dedicate myself to manufacturing and selling. I don't have the time, and for it to be interesting, the price would have to be reasonable, and to be profitable, the price shouldn't be. This is something that should be done by oneself, but I need to think about the best distribution method.
So far, it's worked on all the graphics cards I've tested it on, but I should mention that depending on the resolutions supported by the monitor, the control panel displays different or different resolutions. It doesn't seem to communicate equally with all displays, but that may be due to the ADC protocol and not the adapter. In reality, the adapter doesn't convert anything or modify anything; it simply wires the ADC pins to the HDMI and USB pins. If any graphics card or monitor has problems with this adapter, I suppose it would have also had problems with another external adapter.
As for making an external adapter, that wasn't my initial idea. Besides, I wouldn't be able to get ADC connectors, and we would need +5V, which would have to be "snatched" from a USB port. If a Mac already has few USB ports, it's not a good idea to remove one for the adapter.
P.S. @diehard, changing the subject, do you happen to have a link to that OS9 image compatible with the 1GHz iMac G4? I haven't been able to find it.