This is an awesome implementation!
bluebomber created his own PCB for this (as seen in his video), but there are DIY instructions (circuit diagram & pics) here:
https://ballysternos.github.io
His implementation of the BSOS is slightly customized, so be sure to get the software (including the underlying library) from his repository:
https://github.com/tmurren/bsos-flash-gordon-2021

Latest games would be Stern MPU-200 (1982) like
Catacomb
Orbiter 1

or Bally -35 (1984) like
Kings of Steel
Eight Ball Deluxe

This hardware will work with any -17, MPU-100, -35, or MPU-200 board, but those are examples of the newest. Flash Gordon & Meteor are the newest machines rewritten so far.

I built a prototype and wrote some monitoring/diagnostics for the ESP32-S2-SAOLA.
My experience wasn't great. Maybe I should have stayed with a more seasoned ESP32. At the time I was messing around, I had to side-load the support into the Arduino IDE and the board would sometimes get into a reboot loop that could only be fixed by holding the Boot0 and Reset button and reprogramming.

Probably my fault for poor board choice.
That said, I have code that uses ESP port definitions if anyone wants to blaze that path. There are a ton of advantages - almost endless code size, much more speed, WiFi (as you mentioned), and it could control all the bus lines instead of just the few I'm currently using.

Another fun thing I was messing with - I halted the 6800 while it was running, messed with the CRAM, and then rebooted the processor again. As you were saying, this could be used to change high-score, number of credits, etc. on the MPU running the original code, all via WiFi.

I haven't experimented with any William's boards yet. So far, this is just a Bally/Stern project.

Send me a link to one you recommend and I'll put up some code for it.

tktlwyr - are you still having this issue?
geeteoh - bluebomber and I worked on this a good bit to get it working with the original S&T (based original guidance from you and Quench and the attached disassembly). Here's the process we settled on:
1) Turn off interrupts
2) Get the old solenoid control byte (from U11 Port B)
3) Write back (solenoid control byte) | 0x0F to turn off momentary solenoids & make sure CB2 is low (to select solenoid board)
4) Delay for 32 microseconds
5) Turn solenoid select line (CB2) high
6) Write lower nibble to U11 Port B (lines D0-D3)
7) Delay 138 microseconds
8) Write upper nibble to U11 Port B (lines D0-D3)
9) Delay 145 microseconds
10) Restore original solenoid control byte
11) Put CB2 back to low

@geetoh, do you recommend we try a longer delay in step 7 or step 9 maybe?
In the past when we've missed the high nibble timing, we still got some output. Not sure why there is none, unless we're accidentally always hitting dead space in the possible values. If your replacement board is reading after the original sound byte was restored, it would most of the time get 0xF?, right? Seems like something should happen.

Based on your "startsound" routine above, the big difference seems to be the delay at the end.

Screen_Shot_2021-04-23_at_7.00.15_PM (resized).png

Okay -- easy to try a longer delay. I can put it on a define to make it configurable as well.
If tktlwyr can reflash the Arduino easily enough, perhaps they can test for us.

tktlwyr , did you compile and load the code yourself, or did you buy a pre-burned kit? If you loaded it yourself, can you run an experiment?
In your BallySternOS.cpp, search for these lines:
// wait 76 microseconds
delayMicroseconds(145);

and change the 145 to 250, so it reads:
// wait 76 microseconds
delayMicroseconds(250);

Then, re-upload to your Arduino and give it a shot?
As you can tell from the comment above ("wait 76 microseconds") this value has been in flux already. Hopefully we can find a delay that works for everyone.

That has been reported by a couple of other people as well.
I'm still trying to reproduce / diagnose the issue.
How long will it stay in attract before starting a game?
Do you notice any other behavior than the previous code? Flickering displays, etc.?