Ignoramus335 wrote:
I was thinking overnight. If the issue is fatigue from vibration, what
if I take the motor and pump off, set them up somewhere else, weld and
test the tank, and use it as air receiver tank at a slightly lower
pressure (say, 120-150 PSI).

Then further fatigue will not be an issue, as there will not be
further vibration.
It's not fatigue from the pump vibration, it is
fatigue from the expansion of the tank under air
pressure. Every time you pump up the tank, it
expands a few thousandths of an inch. Any
weakness or corrosion concentrates that stress.

Jon