66 dart LED prototype working for now

About the controller. I use Atmel AVR, for your application the tiny series may be a good choice. Raw PWM sets a duty cycle to provide an average voltage. This works well for motors and solenoids, however will not work well for LEDs. The LED current vs voltage is very sensitive to voltage. A small increase in voltage creates a large increase in current. The circuit needs to be designed to provide good resolution for the desired current range. I have good ideas how to make the control work correctly.

Your recent post suggests the range from a few to 70 mA. This range should be easy to do. I also need to consider how the LEDs are connected in strings of series and perhaps parallel. It may be necessary to to provide separate control for each series string.