>I’m still looking to better understand how ambient liquid pressure affects bubble velocity.

Stokes law

In a stable volume the pressure wouldn't affect the rate at which it 'fell' in this case falling is negative. But in the case of a bubble, it's pressure is equal to the liquid at the level it is in. But as the bubble ascends through the liquid column, the bubble diameter will increase to maintain pressure.

Now if we take Stokes law and the fact that pressure will vary linearly through the column, but velocity will vary by the square, and terminal velocity will vary by the cube, Id venture to say that the optimal water depth is farthest away from the point where the bubble was created, as R would be the greatest (until structure is no longer supported).