Very interesting topic. Thanks for sharing your experience. I also find the options in Python overwhelming. I also am not satisfied with the auto-wrappers like f2py which can be confusing to use or are tailored to a particular Fortran processor.
When you say “callbacks and closures”, are you referring to the Python and Julia side? Fortran doesn’t have closures in the full sense of the word.
Concerning trampoline flags, are you are referring to -ftrampoline-impl, discussed previously here: Implementation of a parametrized objective function without using module variables or internal subroutines - #41 by ivanpribec ?