Dash man
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
The principle behind Roth Conversions is to equalize your marginal tax bracket over time - that avoids high tax brackets by also avoiding low ones. It's playing on the convexity ("progressivity") of the tax code. This is well trod ground and what you are doing doesn't sound anywhere close to optimum.
My wild guess is your tool is not figuring taxes on the taxable account correctly, perhaps taxing capital gains as ordinary income.
I suggest you invest in a commercial tool like Pralana Gold, MaxiFi or Income Strategy, spending a hundred bucks could save you hundreds of thousands.
Pralana Gold projects I’ll save $1.4M in taxes over our lifetime and I’m following the plan it provided. Income Strategy also shows significant savings. The principle behind Roth conversions is not to equalize your marginal tax bracket, but to minimize taxes over the long term. Your wild guess is wrong.