I have found the Retiree Portfolio Model spreadsheet from the Bogleheads to be comprehensive for projecting fixed income sources (SS, pensions, SPI, etc), investment income (dividends, LTCGs), and the resulting taxes (Federal, State, IRMAA).
It takes awhile to understand the inputs, graphs, and outputs. I find it to be very comprehensive, and effective for projecting Roth conversions. The outputs show results with and without Roth conversions, and the percentage of filling up each tax brackets during the conversion period and RMDs.
The authors put a ton of work into this over many years, and the calcs seem to be well vetted by many users.
Set aside a few hours to check it out, I think you'll be impressed.
https://www.bogleheads.org/wiki/Retiree_Portfolio_Model