You are completely ignoring risk. Firecalc assumes future performance will mirror past performance.
Of course this isn’t zero risk. The OP knows that and I’m guessing that’s why he was asking for our opinions. But if done right, the risk is manageable.
As for using Firecalc averages, we use the best data that we have. I’m sure that OPs results will be different than the average, but just eyeballing those numbers I think something would have to go severely wrong economically to not come out ahead.
For me, I believe having a small amount of debt to be beneficial and I doubt I’ll ever be debt free. Especially if you can lock it in at historically low interest rates.