Hi David,
ok, got it running, Problem with GMIC plug-in for Gimp 2.9 pointed me in the right direction.
If you are running gimp from the distribution and your own compiled version in parallel you need to set the LD_LIBRARY_PATH accordingly.
I created a short summary at https://opensource.photography/content/compiling-gmic-linux-mint-debian-edition-2 including back links to pixls.us.
@David_Tschumperle: I hope it’s ok to reuse your mascot as teaser image 
Cheers,
Martin