Great post! I suspect that PYTHONPATH hack might be useful in reorganizing a particular repo at $JOB that contains a few different deployable packages and a common library.
The thing I like most about using Makefiles in this way is that it can provide a consistent dev experience across many repos in a team setting, if each repo defines a consistent set of make targets: make setup, make test, make build etc. I don't have to care too much if the project is using pip vs. poetry, or pytest vs. unittest.