Have you considered a configuration based on where the boards need to be located.
Quoted from BENETNATH:
* Coils : 4 flippers (double wiring), 2 slingshots, 3 bumpers, 1 ball tray, 1 knocker, 3 chime, 16 coils for the scoring system in the back box
TOTAL : 30 coils (34 including the double wiring, as it seems that each flipper coil shall be considered as 2 coils ( 1 for action, 1 for hold)
* Switches : 1 ball tray, 2 flipper buttons, 1 start, 4 slingshots, 6 lanes, 10 rollover, 9 targets, 3 tilts, 3 bumpers
TOTAL : 39 switches
* Lights : 12 GI, 31 active + 21 active in backbox
Starting with solenoids: There are 16 coils for the scoring system which are all in the back box. That can be realized using two STM32 cards full up with solenoid wings. That also gives 16 direct inputs for figuring out things like when the score reels are in the zero position, and error detection, such I pulsed this reel 10 times, but I still haven't seen the carry bit, etc. Not sure what is supported in MPF.
Next are the 3 coils for the chimes, plus the 1 coil for the knocker. Nice to connect together all these guys together on a single STM32 because it is all part of the cabinet. That also gives inputs for the 1 start and 3 tilts. Typically you'll have some incandescents in the cabinet for the coin illumination, so it would be nice to have an incandescent wing down there to make wiring as easy as possible.
So the remaining coils on the playfield itself contains 1 ball tray, 4 flippers (8 solenoids), 2 slingshots, 3 pops. So that is another full STM32 to run all that. If interested, these can be mounted on the bottom of the playfield itself if there is space to fit it. It can make sense to split it into 2 STM32 (such as all the flippers on one towards the bottom, with two incandescent boards to run the active lights at the bottom of the playfield, then a second STM32 towards the top for the ball tray, 2 slingshots and 3 pops + another two incandescent boards towards the top). It seems with all the rollovers there might need to be one more STM32 for the extra inputs on the playfield.
For the two machines I've swapped over, I created a quick sketch of the general locations of STM32s under the playfield. This allowed me to minimize wiring on the bottom of the playfield which makes it much easier to keep things "clean".
Good gosh, I should have looked up the pinball machine earlier. That age of William machines I believe were dual wound coils, but there is an EOS on the flipper where it automatically disables the high power coil. If that is the case, you only need a single solenoid for each of the flippers. There is no need to control the two coils for each flipper individually. Also, with that age of game, I'm guessing that a single solenoid powers both of the left flippers and another single solenoid powers both of the right flippers. If that is the case, it moves the 8 solenoid requirement for the flippers down to only 2 solenoids for the flippers.
Bah, wife is telling me it is time to go to bed. Sorry, about the brevity and scattered-ness of the message. I haven't had any time to respond to many of your questions. Neopixels are coming, but they aren't there yet with OPP.