The MOT_THST_HOVER is calculated by the drone in flight automatically if you have MOT_HOVER_LEARN activated. Even if you change the payload the drone will adjust the MOT_THSH_HVER value to the required one. So, you don’t really need to worry about this.
The MOT_THST_EXPO parameter is different, as you say you have to derive its value from a graph of approximations according to the propeller size. This works very well for most multi-copters, especially small and medium sized ones. But in some cases, with large drones or drones that use “closed” ESCs which may be correcting this exponential, it is good to adjust the value.
Here are two forum posts on this same subject for you to take a look at:
As for how to calculate the MOT_THST_EXPO parameter from the thrust stand data. There are several methods but the simplest is to use the following spreadsheet. Just by entering the data it automatically calculates the correct value.
Finally, as for the stand you attach the biggest problem, I see is that it does not have a data output. The only interface is a small screen. This is ok to see maximum thrust or thrust at some point of the throttle, but it makes it very difficult to get data from the whole thrust curve. The only way is to do it manually by writing down the thrust at each throttle value.
As I said in a post I attached above, I would recommend that you try to make one yourself if you can. It is the cheapest option and allows you to automate the tests and capture the data from a computer.