Replace any custom PCB with off the shelf breakout boards. Redesign the enclosure so the breakout boards can be mounted. Instead of a custom motherboard PCB, solder wires between the pins of each breakout board, sprinkling through hole resistors and capacitors around where needed.

Since I didn't go down that route, I don't jave any recommendations for breakout boards that could do the job. I'm also not sure if the assembly is any easier than the assembly of my design.