In all seriousness, and even with all the technical workarounds, handlebar height is such an important aspect of fit, that I am having difficulty with the idea that the bike can fit well "except for the height of the handle bars" as you wrote.
It's sort of like buying shoes that are a little too short or too narrow because they look sleek and are (perhaps a factor here too) on sale. If it were a used/rare bike, I could understand, but there are so many new bikes out there to choose from, why not choose exactly the right one?
Plus, it's not a good start with a new bike to be purchased with the intent of having to make significant changes to it. Maybe you like the handling of the Alpenchallenge you have test ridden, but the handling will change with whatever solution you try here. Perhaps you like the sleek look, but that will certainly be gone with a steering tube extender or adjustable stem.
There are companies that build bikes that might better suit you. I don't think Marin has been mentioned, but in addition to their hybrids and fitness bikes, they have "flat bar gravel bikes" (ugh, don't let's start on that market category . . . this is just an example, not an endorsement) that have at least level handlebar-saddle relationships. And are price competitive with the Alpenchallenge:
