"But what happens if the original carburetor is still set to open 100% at full throttle? Should the engine die, because it gets flooded with fuel? "

The good news is you need more load.

You will usually develop more horsepower with the stock carb than with a smaller carb, even if it's not open 100%.
But the transition should be better with the smaller carb.

I know it's not easy for you to check rpm and load.

For most of us we simply change props and check the rpm and the color of the exhaust.

By the way, I have really enjoyed this thread.