perhaps we should cure the problem in stead of address the symptoms? the only thing remaining as problem sources in your system is then:
1) the head that the pump faces as the tank is so far below the pumper carb
2) the pumper carb
since we can't change #1 that's a non-starter.
the only thing would be to change carb types with one with a better impulse pump. could it be the pump itself, when facing the pressure head it has to suck causes back pressure on the one side of the pump cycle and it causes the gas to dance up and down a bit in the line causing bubbles?
is this why when the electric fuel pump is on right outside the tank, pressurizing the line, the problem disappears?
were all sources of gas line vibration eliminated?
should we try a rotax 185 carb on your solo 210 and see what happens?
T.