There is also https://software.sil.org/ukelele/ which is a
[…] Unicode Keyboard Layout Editor for the macOS
Screenshot from their website:
As the problem owner does not want to write greek proze, but wants to use (greek) symbols to express math in a programming language, he could try use Ukele to modify/augment/re-map his current US keyboard with extra math symbols, so he does not need to change language every time he switches from math-mode to Python or Julia or whatever. This also allows for mapping other math symbols, like ∂ ∜ or ≨, symbols that the Greek community have not added to their alphabet.