Since cutting_fun is function that only exists in Python, unfortunately you can’t call it directly from an expression of TTree::Draw.
You can write the function in C++ yourself, as you mentioned, or try with NumbaDeclare, e.g. see this tutorial:
In short, you decorate your Python function and, under the hood, a C++ wrapper for it is generated so you can call it as Numba::cutting_fun in your case.