FIGURED IT OUT. indeed, it was the matter of adding the right binary to the correct package manually (it didnt work when i just tried to add it straight to '.', i had to manually add it to casadi package).

this was done by adding the following line to .spec file under Analysis config:
binaries=[('venv/Lib/site-packages/casadi/_casadi.pyd','casadi')]

(.pyd is like a DLL designed for python)
thanks to @furas for pointing this out!