After enlarging the photo I can see you are correct. The link simply shows that a smaller airframe of basic plan form can achieve what you are after as far as altitude. Your idea of a larger plan form should also work as in the second link.

From your posts two different prop/motor combinations would be ideal. Would it be possible to use the Tigershark type of plan form with both a tractor and pusher engine. The pusher could be propped for lower altitudes and the tractor could be propped for high altitudes both with folding props. Engines could be individually selected or even run in combination. Boom diameter could be increased in diameter to contain the entire payload you need plus easy retracts. You could even make the low altitude engine fuel burning to get to altitude and save the batteries for high altitude work.

If in fact you would consider the twin boom as in your previous posts you could outfit both electric motors with the high altitude props and still use an internal combustion engine as a pusher to get the airframe to altitude before engaging the electrics.

I find what you are doing interesting however if I have nothing of value to contribute to the topic please pm me and I will stop.

Dennis