Update:
I found some hints in /usr/bin/dump-stack and created two files in /mnt/us/ :
DISABLE_CORE_DUMP and DISABLE_CORE_DUMP_ALERT
No more heavy dump files!
Only things that are created from time to time are Indexer_ files and some small html and tar.gz files.
This usually happens after Kindle UI freezes for a minute or so, after I enable Wi-Fi. I suspect that it freezes when trying to check for OTA, and I blocked OTA binaries (and otaup script). So maybe because of that
All this happened on FW 5.16.21
I’ll try to replace ota scripts with some dummy empty scripts later, so that command “will work” keeping Kindle UI happy.