I don’t think you’re being totally impartial if you suggest that Donald Trump and Kamala Harris already agreed to the terms of any upcoming debate. Is Harris still going to show up to debate JD Vance as previously agreed to? Of course not. The whole election changed because the Democrats had a vegetable at the top of the ticket who was getting his butt kicked, so he got replaced.
I feel sure there will be at least one debate between Harris and Trump, but who knows in this environment. If anything though, Trump is overconfident, not scared, in any given situation.