Maybe it's time to look towards a different camera.
You can't expect a dump truck to fly no matter how long a solution takes, Re: "Might need a firmware update though." and 4K really isn't 4K if it goes into crop mode Re: "Involves 10% crop".
Well thought out and documented points brought up here but in the end It might be time to grasp that you can't plow 20 acres with a butter knife if "4K, 120, 4.2.2 can't be compromised" and find a different device to get you to where you need to be. Seems to me 7Siii can't reasonably hit all the points you require unless you start throwing money at it and tacking on additional hardware.
This isn't the first time 4k, 120, 4.2.2 has been shot by anyone has it? It begs the questions 1) How was it done before now? and 2) What was used to capture it? If you need to fly trying to do so in a dump truck merely because you have one isn't the best or only option. It's like making the introduction, "Mr. Square Peg meet Mr. Round Hole."