I posted the full solution here : https://github.com/armbian/build/issues/1582
The basic idea is :
Once the "--delete" issue is corrected, log.hdd is better, but there is still an issue : the rotated files overlap, because the effect of the rotation is not mirrored to /var/log. I have a log rotated daily, but each rotated file contains 3 or 4 days.
This can be solved easily by the following process :
ramlog write (to update log.hdd before rotation)
ramlog postrotate (which will copy to /var/log the modified files of /ram/log.hdd).
See the code on Github, link above.