I need some help/guidance here with a challenge at work. Hopefully y'all can give me some direction here.

I have to borrow $X and I have several lenders to choose from.

Each lender charges a different interest rate.

Most, but not all, lenders will charge a penalty for any shortfall in usage below a certain level.

Some lenders however will give a rebate if usage is over a certain level.

And an added wrinkle is I need to keep a certain token usage level with each lender for relationship purposes.

Currently this is allocated with a general approach to avoid the non-usage penalties and then allocate to the cheapest interest rates. I'd like to build something that could quantify the optimal approach, if only to then quantify the cost to us from deviating from optimal.

What tools should I be looking at?

I'm sure you could write a program to do this in Matlab or python.

You can do this is excel. Just build a model year by year (or month by month) and setup assumptions for your line and f credit usage which would kick off fees or rebates. Run a number of sensitivities on your assumptions and that should hopefully point you in the right direction. You could bring this all back to a current day NPV to try and rank the lending options being considered.