German,
Not sure if you've got anywhere with this - I'd seen the same thing with memory increasing and eventually KStars crashes mid-imaging. In summary, RPi 4 4GB, + astroberry, Canon DSLR, AZEQ6 & ZWO 120mm guidecam.
Tracked things down, by several simulation tests, wrote shell scripts to output memory usage and noted the cache seems to fill up, and eventually starts using Swap memory.
By updating the script to watch out for the cache and available memory, and if it gets too full, then things get purged automatically.
Memory usage keeps rising, but the script keeps purging, so I'm hoping I have found a workaround rather than solution.
My post here shows my extensive tests, and scripts also so you could try and see if you got same result/success?
Post back so we can see!

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