GARCH is a typical model for volatility and Cox-Ingersoll is the one for interest rates. I change them independently of each other to test the robustness of strategies under different market conditions.
i am familiar with GARCH and with Cox-Ingersoll (but only a little bit).
At the end of the day you have to predict a market event (rates will go down) and then predict how other factors will respond to that. The best way to predict is to see what happened in the past.
You can shock your book to see how different scenarios can play out. But history is a pretty good guide of the future.
(This is coming from a guy who neither backtests nor simulates strategies)