I’ve been digging further on this issue, as my system is expanded already.
It seems the shrinking how-to’s and scripts are working for the standard 2 partition images only.
M2P has 3 partitions on the SD, probably because it is based on the NOOBS framework?!
I’m not sure why this approach was chosen. Is it necessary to have NOOBS underneath?
I found a thread how to get rid of the NOOBS, but I wonder if that will
affect other M2P functionality somehow (updates, configs, etc.)