But this, indeed, is quite an expensive endeavour so i definitely should try it out with an older model first. I am gonna keep searching for someone who could help me copy the PCB in the meantime and i'll update this if i find any useful information on it. Thanks!
Another member(he can reveal himself if he wants to) and I were contacted by a Chinese manufacturer who did something almost identical to what you are trying to archive, they made their own hash boards which were more efficient than the regular S9 hash boards, and their own firmware which you had to deploy on the default controller of the S9 (the hash boards won't work with the default firmware).
The company wanted economical consultation in terms of pricing, I contacted a few major resellers in China, Russia, and the U.S, and sadly the final sale price was not tempting enough to convince anyone to get those hash boards, mind you the company that did this is located in china, and they paid us just for the consultation, they clearly had the money and the manpower needed for that job, and the results were not so great.
I am not aware of where did they go wrong or how difficult it was, but I am certain they had more skills, money, and resources than you do, which is one of the main reasons why I told you in my first comment that this "won't work", it wasn't a blind baseless statement, I hope the above information will be beneficial to you oneway or the other.
Good luck.
Its obvious that the custom made hash boards wont hold as much value as the original hash boards if you're trying to sell them. Because essentially what you're doing is substituting the physical labor, time and effort that you are going to put into assembling the thing to save yourself some cash which you woud need to buy a new hash board (in my case anyway, but i am sure it was the same for that company).
Mass producing and selling custom made hash boards is gonna be way less profitable than making a few of them for yourself to use for mining because, as you said, people would rather go for the original ones since the price isnt gonna be much more different.
As for the custom firmware, it was most likely because they produced a modified version of the hash board to run on the control board for the S9. I am pretty certain that making a one to one clone of a board will not require a custom firmware to be flashed on the control bobard and it should just work because every single bit of data is freely accessible on the microcontroller.