I am certainly not an expert, but I don't think a bad base gasket has access to your coolant. The one way flow of the pressure seems to confirm it's not subject to crank vacuum.
And, seeing your other recent posts, I'm not sure a water pump seal would explain the bubbles. And with that, we'd expect some coolant loss, I think.
So, I think you are on the right track. At least it's sort of the easiest of the three to check, I guess.
But, I'm just guessing because I can't sleep.
