Little update after a few months for people who stumble upon this thread when they have the same problem:
I worked down the mountain of reviews which happened quite fast at some point, similar to how fast it appeared.
After a few days I disabled ghost reviews like Sidgr recommended. That makes quite a big difference, my review count stayed quite low since and I don’t see any negative side effects. I wonder what’s the benefit of them. I don’t think I’ll ever activate them again.
The most important point seems to be keeping the reviews on a low level. I reduced new cards from 4 or 2 per day to 2 or 1 per day. That also helps understanding cards you have problems with because you have more time to analyze them (my main problem are very similar grammar points and writing them down in groups helps a lot). I have no problem learning 10 words per day but grammar is a different beast.
I wanted to learn as much as possible because I went to Japan for a few weeks in March. But I discovered that while I understood nearly everything people were talking in public (they mostly use really simple grammar) my level dropped below N5 while actually talking to people anyway.
I still hate the hourly reviews though. I really would like to do all cards of one day in one run at a time I choose myself like in Anki. Doing them every day at the same time (as a workaround) is also not a good solution for me.