The interesting part to me is designing and building new and innovative devices
I can relate. Development is a lot more fun than production and admin, let alone finance.
Why do you feel a "cruise control" for diving would be cumbersome? Is that based on your knowledge of prior art?....
The problem is controlling displacement in any attitude. Sensors are a minor part of the problem, though precision is on the low side in shallow water. The simplest solution is to use hard tanks like a submarine because you dont have to constantly compensate for gas compression. In practice, the weight and power requirements are prohibitive. Even then there are attitude issues.
Just getting batteries to work reliably in salt water is a challenge. Now add cables (solenoid) or hoses (pilot) to dump gas at whatever point happens to be the highest at the moment. Again, a hard tank makes it simpler. You really dont want to blow excessive gas constantly as the diver meanders up and down.
I have designed automated analog and digital decompression systems for saturation diving complexes and studied/borrowed related techniques a much simpler task than you propose. The physics starts to make it cumbersome pretty fast. Start with your maximum buoyancy compensation range. Take twin 100 Ft³ cylinders and a 7mm wetsuit. That would be about 15 Lbs of air weight as the cylinders empty and perhaps 15 Lbs of displacement loss due to suit compression. Thirty pounds of displacement is a little under ½ Ft³ of displacement. A hard tank would start getting pretty bulky and expensive.
Controlling a flexible bladder like a jacket BC or wing not only introduces dramatically more cycles to the mechanism but magnifies the problem of getting your dump valve to pull gas from the high point without the diver cooperating by changing position. It gets into the whole personal preference issue of inflation bladder geometry, which rivals religious debate on this forum anyway.
You also have to tune the system down, or provide a counterlung, so breathing cycles dont cycle your system. A large guy at moderate work will move 3-4 liters per cycle. That is ~6.6 to 8.8 Lbs of displacement change. However, if you cant keep the customer at +/- a pound I suspect they wont be satisfied with the product. We havent even cracked ascent and descent rates.
As far as descent rates, what would you use? I know I am really old school but I duck-dive from the surface and descend a 120 FPM in Scuba (200 FPM freediving). You could limit ascent to 30 FPM, which drives valve sizing.
I always enjoy design reviews. I hope you have ideas that will make me slap my forehead in one of those why didnt I think of that salutes.