I’m normally into the homebrew NES articles but this has no details and no links to details I can find so I’m inclined to vote blogspam.

Is there a ROM, a write up, anything? Nevertheless it’s impressive to ship for NES / FAMICOM and the GameBoy. Was the source all the same? I’d love to read more from OP.

Pretty much. No info about the self-publishing process at all.

I've seen many use NESmaker and sell physical copies. The NESmaker website has the software to create NES games and they also sell hardware to write to cartridges too:

https://www.thenew8bitheroes.com/

Then I think the creator of NESmaker has some connection with Limited Run Games where the more polished NESmaker games has been published via LRG.

https://limitedrungames.com/collections/nes