It would be lovely to have proper and working self-contained app again

Looks like there is a bug with cairo, at least.

It just crashes on 10.16 with segfault. Nothing in the debug output yet. It either crashes with dump bellow, or segfaults or crashes with "Assertion failed: (!surface->finished), function _cairo_surface_release_source_image, file cairo-surface.c, line 2092." Process: python [84757] Path: /...

Do you have your for available somewhere on github?
If not, please, can you share?
Your build doesn't work on Big Sur, and I'd like to rebuild it, without going into duplication of effort in patching etc.