Run a septic dye test? That would have given you an indication of how the leach field effluent was being dissipated.
I don’t do a septic dye test, I think they are more beneficial in determining if a septic system has been bypassed. If I saw pvc in a weird spot I would then consider it.
I would not do a hydraulic load test on a vacant house. The drain field has dried out and will absorb your test water. Back when I did septic inspections Missouri you couldn’t do that test if the house has been vacant more than 60 days
I was out there again this morning to retrieve my radon monitor, and measured the effluent depth again out of curiosity. It was was sitting at 88" from the top of riser, I think it is fairly obvious there is a problem.
Failing drainfield, restricted field causing slow percolation. Your introduction of 240 gallons flooded the system, and it eventually perc’ed back to normal, preflooded level.
Many reasons will fail a septic inspection:
The baffle needs repairing
Tree roots are damaging the soil around the drain field
House drains are emptying slowly or not at all
Sewage backs up into the house
The grass near the septic tank is greener and wetter than the rest of your lawn