There are working reverse engineered solutions for Nvidia graphics but almost nothing for baseband. Do you think its because there’s no demand for it?
I do 
The demand is currently not significantly higher than 3000 devices. Not by orders of magnitude.
Not only is a non-baseband version not cheaper, it is also not more secure. There is no difference between a turned-off baseband and a non-existent baseband. (You can even screw it open, remove the wire between baseband and switch and it can never be on) Also, in which way do you think it could spy on you if you were to turn it on? It does not have access to any of the other components. The only thing it can do is to reveal the position of the device. I’ve written more on that here.
Manufacturers don’t need to release source code, but chip specs. That’s something very different. Now, if you fear that goverments order builtin backdoors, which gov do you fear? US? Each where the chip maker has an office? The one where the chips are actually made?
And why do you fear the capabilities of a chip that a) can be turned off completely b) has no access to the rest of the system?
You should be more worried about backdoors (potentially) implemented in the main CPU. ARM (or the iMX series) is not an open design. But even if it were: How could you establish that the design you have is the one that actually was used to create the chip, without modifications? We are far away from the ideal you are looking for. ASAP could mean a pretty long time.
Software design is not simpler, the telephony stack is already there.