... it's a mess to calculate.
Yes, even assuming we know future tax law, in addition to other unknowns such as life expectancy and investment returns.
There is a long thread on this subject at
Retirement Tax Planning - Income Optimization? - Early Retirement & Financial Independence Community. I tried the free version of the tool mentioned there and it was worth what it cost. The claims for the version behind the paywall, however, are interesting - but I haven't decided whether to try it.
FWIW, below is what we are doing with some of the available tools. While there are differences, our situation and yours appear very similar in complexity. :)
Open Social Security: Free, Open-Source Social Security Calculator - this suggests optimum SS benefit start dates for each of us. As we have no reason to expect shorter than average lifespans, and noting the marginal tax rate implications of any extra income on Roth conversions, we treat the results as "start no earlier than ___." Might just do FRA for the lower earner and age 70 for the higher earner. Fixing the SS start dates removes two degrees of freedom.
Optimal Retirement Planner - Extended Parameter Form - this gives us a general idea of how aggressive we might be with Roth conversions. We take the results with a few grains of salt, however, when it suggests large conversions that would trigger NIIT (I-ORP ignores NIIT completely) and IRMAA (I-ORP recently added some IRMAA considerations, but perhaps doesn't include the full impact). On the other hand, there's always the chance one of dies prematurely and the survivor filing single would wish we had done larger conversions. Also, "tie (or even a slightly lower tax rate later) goes to the Roth"
when one uses taxable funds to pay the tax on a conversion.
Considering the last two sentences, it may make sense to go "one bracket higher" when converting for MFJ if the next higher bracket is "close" (e.g., 10% to 12%, or 22% to 24%) vs. the expected withdrawal marginal rate. It's harder to see converting into the higher bracket for the larger steps of 12% to 22% or 24% to 32%.
Case study spreadsheet (CSS) - No surprise here, eh? Actually we use our own tax spreadsheet, built over many years for our specific situation, but it has the same functionality of the CSS's marginal tax rate chart showing all the steps and spikes for a range of capital gains, traditional to Roth conversions, etc. Given the same inputs, the CSS and our own (and TurboTax) all calculate the same federal tax. The CSS is slightly less accurate on the absolute amount of state tax, but exact for the marginal rates of interest.