I tried to search for this, but the only results I got were backtesting Leveraged ETFs, which is not what I’m looking for.
Does anyone have any data or research which backtests investing into the SP500 ETF with 10, 20, 30% portfolio leverage (available on IBKR for example). Taking into account the interest rate of borrowing? Or just any online calculator where we can play around with the leverage and the interest rates?
Couldn’t you just plug in e.g. 10k, 11k, 12k, 13k as investment amounts, and compare? 
Ah the interest rate - you’d need to do the rest in your excel sheet indeed.
I think a semi-automated tool would hardly exist because the borrowing rate is likely not historically available in databases and would differ lender to lender.
There are “forward looking” calculators for investing with leverage, but haven’t found any backtesting ones.
Most backtesting tools should be able to this? For example testfol.io.
They are not as flexible as your own excel, or python script. But time-interval rebalancing should be sufficient to compare allocations. But most such tools don’t care about margin requirements, so be careful.
You also have to bring your own assumptions for expenses and markups, if you don’t backtest a real fund. In the example above, I used CASHX?E=-1. This is the short term rates with an expense markup of 1 percentage point.