I second the recommendation of the compute module. You can't get a 500-pcs production using what RPT uses for a reasonable cost. They produced several million boards. Just flip the board upside down and inspect all the 0201 resistors, amazing, right? What you can't see is the ball grid array under the main processor. Also I've failed to find raspberry pi design files to even modify it. For something hundreds per year, I would use a compute module and only have to deal with the motherboard design, which will be a lot less complex and higher yield in production. More people will be able and willing to design for you for reasonable compensation.
Arduino data loggers, user interface, printed circuit board designer since 2009, RPI 3B 2B 2B Zero Jessie, assembly/C/C++/java/python programmer since the 80's