You are on the right track I think.
It takes some time to get the efficiency and smooth jibes with a board that has less than 4 fins. It's true there are times you go upwind much better wth the board flatter, especially in light wind. But with surfboards everything is often more of a balance, more than a shift between gears.
Do try pushing your back foot position forward to just in front of the tail pad. You really only want the tail pad for turning on a wave. Back foot should be just over the front if the front fins for upwind/ light wind. Ideally front foot will be near the wide point of the board or slightly forward.
Get to know your board with this positioning and then move your feet closer and closer together. When they are almost together look down, between them is the center of pressure of the board. If you can ride with equal pressure/spacing forward and aft of this point you will stop falling. Eventually.
Your fin idea sounds ok. In general the trailer in thruster sets slows kiteboard down and sometimes hurts upwind, because it is not at the right angle to drive off the rail upwind.
I choose to use bigger front fins and a small trailer - good compromise.
Also like twin fins , no trailer, easy to tack and loose but harder to jibe.
Another fin option you could try is the MR twin plus trailer.