what I did now because of ZRAM link (ZRAM status):
I increased in /etc/ztab my mem_limit from 150M to 350M (Raspi 4)
stopping service before, then editing, then restarting again - and for safety reasons: restart raspi again.
after that at least the zramctl output tells me that my max-size is now 350m instead of 150m
I checked my /var/log/openhab directory (du -h) and it tells me that my openhab.log* and events.log* have a size of 140M (don’t know exactly what of these old files is also loaded into zram…)
[08:02:56] openhabian@openhab:/var/log/openhab$
[08:02:56] openhabian@openhab:/var/log/openhab$ zramctl --output-all
NAME DISKSIZE DATA COMPR ALGORITHM STREAMS ZERO-PAGES TOTAL MEM-LIMIT MEM-USED MIGRATED MOUNTPOINT
/dev/zram1 500M 214M 45,2M lzo-rle 4 3862 99,5M 350M 99,5M 0B /opt/zram/zram1
/dev/zram0 600M 4K 87B lzo-rle 4 0 4K 200M 4K 0B [SWAP]
[08:03:12] openhabian@openhab:/var/log/openhab$
[08:03:28] openhabian@openhab:/var/log/openhab$ df -h
Dateisystem Größe Benutzt Verf. Verw% Eingehängt auf
/dev/root 29G 9,2G 19G 33% /
devtmpfs 1,8G 0 1,8G 0% /dev
tmpfs 1,9G 0 1,9G 0% /dev/shm
tmpfs 1,9G 1,4M 1,9G 1% /run
tmpfs 5,0M 4,0K 5,0M 1% /run/lock
tmpfs 1,9G 0 1,9G 0% /sys/fs/cgroup
/dev/mmcblk0p1 253M 48M 205M 19% /boot
/dev/sda1 29G 23G 4,6G 84% /media/usbstick
tmpfs 388M 0 388M 0% /run/user/1000
/dev/zram1 469M 199M 235M 46% /opt/zram/zram1
overlay1 469M 199M 235M 46% /var/log
[08:03:31] openhabian@openhab:/var/log/openhab$ du -h
141M .
[08:03:35] openhabian@openhab:/var/log/openhab$ pwd
/var/log/openhab
[08:03:36] openhabian@openhab:/var/log/openhab$
I will give an update if everything is still working in the next days.
@mstormi : please tell me if I made some basic thinking-errors 
thx