Approach it from the other direction. Make a way to adjust the propane to the available air quantity, say with some kind of metering valve, with a fine adjustment capability. Easier than making different size holes in the pipe to ingest varying amounts of air.

I might try some way of dropping the pressure in the exh pipe, like an increase in pipe diameter, with the air intake holes just downstream from the change point. Tapered might be better than a sudden transition, but I'm guessing.

Not responsible for advice not taken...