I often wonder why people think they have to start from scratch and build an entire printer. Few people's printer problems are printer problems. They're usually problems engineered into the printer at the firmware level. The stuff that actually does the printing is dumb components that do whatever you tell them and mechanical engineering someone else has already done for you. Right down to the commercially available connectors that connect the dumb components to the broken-by-design control board.

Why remake the entire printer instead of just the control board?

Not to mention, you can add features that should be there on every printer but that no manufacturer has considered including. Like an emergency stop feature for when the printer gets a corrupted print job and starts printing out as many blank pages as it can with the occasional page with a single line of gibberish. Tell it to stop, and it actually stops and spits out whatever sheet of paper it's working on at the time. No holding down the power button. No clearing the jam that results. No uselessly canceling the job at the source. No questions asked. Just stop printing, clear the paper path, ignore the rest of the job, and lie your ass off and say you finished the job so no software gets any funny ideas about resending it.

Pretty sure you can find schematics to make a BT100 printer somewhere ( https://en.wikipedia.org/wiki/BT100 ). You wouldn't want to, but you can.

More seriously, I think the main problem is simply achieving a usable resolution for a 3D printer is way easier than for a 2D one. Hence why the real hard part (the print head) is using an HP cartridge instead of rolling out their own here. Given my quick googling, it's usually that or building an inaccurate pen plotter and calling it a printer...