For anyone else interested, I have four different operating systems on my newest desktop computer: Windows 10 Home, Debian Buster 10, AV Linux 2018, and Fedora Design Suite 30. Have tested the MyPaint appImage 2.0. Works on Debian, Windows, and Fedora. Does not work on AV Linux yet that is a more specialized custom built variant of Debian where I have been able to pick up all the packages necessary with dependencies to install version 1.2 from the Debian website under stable packages.
The same problem you describe here I have experienced with Debian Buster 10 and Fedora Design Suite 30. Dependency conflicts and breaks will not allow you to install MyPaint 1.2 from repositories along with the latest GIMP 2.10 that is really nice having the MyPaint brushes included. Lately I have been using these two graphic arts programs more than any others.
Strangely enough Windows 10 will allow you to install any MyPaint version or GIMP with no problems or conflicts. I write strangely here since Windows is a proprietary software operating system. Linux operating systems are more customized for open source software like the GIMP and MyPaint. Despite all the recent criticism towards Windows operating systems I find their latest version 10 to be more reliable and less problems than any other operating system. Especially with file sharing on a closed network between computers, nfs or samba for Linux far more difficult to use.
