I am planning on duplicating your mod in my own rapidstrike within the next month or so. If you want/need a beta tester, your kit would save me a lot of time. I am extremely tech savy, and have a friend who is arduino experienced and a very good coder who is willing to help me with the project. I would be very interested in buying a kit from you, even if it's in an alpha or beta state.
If you want to add smartphone control, you can do it the easy way over bluetooth. If you add a bluetooth UART from Adafruit you can use their existing Android/IOS app to send data back and forth.
Love your build. Started doing the same thing to mine as well. Built mine on a single board like this:
But once installed so the displays are on the right side of the gun, i found it was slightly too thick and was interfering with the magazine.
So ive since made a far smaller version:
My magazine detection is using switches to detect the ridges on the sides of the magazines. With a couple of notches its quite simple.
If you're running out of pins on the pro mini, and dont want to use a larger arduino, you could always use an i2c multiplexer/extender to let you use more devices.