How to take a raspberry pi project and get it mass produced
Even if you could make the board, you would not be able to get proprietary SoCs to populate it.  You will not be making your own Raspberry Pi. That said, there is a version of the Raspberry Pi designed to be used by Original Equipment Manufacturers (OEMs), but frankly, the price looks ludicrous.

There are similar computers on boards, like the Beaglebone Black, that don't use proprietary chips, that might be more friendly to your project.

If you just need a microcontroller, and not a full computer, your options go way up.