I tried some of the procedures I found on
https://lists.gnu.org/archive/html/lilypond-user/2019-05/msg00202.html, the thread you advised me to look at but they did not help me very much.
So I tried to develop some trick or tweak without having to program things with scheme, with which I have no experience at all.
Anyway, I found something that gave me a result, not quite identical to what I wanted to get, but good enough for me and probably sufficiently clear for a pianist.
See the attached screenshot.
Best regards,
Robert