Change to RollingRandomAccessFile does no harm on my system.

I wanted to try if it really fixes the issue by triggering the zram sync.
You need a very recent openhabian (latest) to get the zsync service installed with zram option.

systemctl start zsync succeeds, but for me the log does not recover (both openhab.log and events.log are affected, openhab-cli console shows new log entries which are not seen in the logs).

I had this on very first attempt, too, but after OH restart it's fine now

Not sure. I tried restarting OH, even clearing the cache, and finally reboot as well. Works until I trigger the sync, from then nothing is logged.

After a few hours there seem to be new log entries, but there is a gap. Maybe we need to lower the cache size...?!

After a few hours there seem to be new log entries, but there is a gap. Maybe we need to lower the cache size...?!

What do you mean by "gap". Are there messages missing or do you actually mean "delay" ?
I wouldn't know how to reduce that. If I understood that stackoverflow comment correctly cache size is 256K fixed and not configurable anyway.
Try producing extra log lines by increasing log level for a test.

Testing shows the change doesn't fix the issue.
I'm still seeing it every day.