Sounds like a tall project. I have done the Virtual Pinball machines and the hybrid pinball machines.
There is a learning curve involved from the Arduino side in regards to building everything from scratch approach.
Using the Arduino Mega and Uno, my hybrid machine utilizes Bally Lamp switch board and Solenoid board.
This way I was not dependent of soldering up my own control boards.

I have enjoyed that journey and found this forum very helpful and supportive.

Alternatively you could take the P-ROC or FAST hardware approach with Mission Pinball Framework. This approach is still challenging but gives you a better beginning point. You still get to do your own rules, choose what type of displays to use (7 Segment or DMD), have lighting effects, and have your own custom sounds. The cool part with this choice is there is hardware and software support.

Good luck in your journey with what ever direction you choose.

One other thought, is to move away from having 6 seven segment displays and go with a single DMD/LED display.

Controlling a DMD is fairly straight forward with an Arduino mega or Teensy 3.5 or 3.6. This would easily be controlled by the MPF solution or your arduino direction.

Having 7 Segment or 14 Segment displays are classic, but you can do so much more with a DMD or small TV screen with cool effects.

The great thing about coding your own is you can do both.
For my close encounters machine I started with the original rules and sound, but that got old quick.

I added skill shots , extraball, specials, and 43 sound effects to events. Using a couple of dip switches your original or new rule set could be easily configurable.

https://pinside.com/pinball/forum/topic/close-encounters-arduino-conversion

Just curious what your plans are as far as the uno triggering the sounds.

What I did is have a raspberry pi receive the triggers to play sound. It enables me to play 5 channels of sound to my speakers plus store a large collection of event sounds to choose from.

Another question, do you plan to use a switch matrix or something else for switch input?

Lots of white wires