Paying for Roth Conversion

I’m 51 and just retired; my husband (50) will work a few more years. Looking for input on paying taxes on Roth conversion. Details * \~$2M+ in traditional 401(k)/IRA * \~$2M+ in taxable brokerage (very low cost basis) * Existing Roth balance with sufficient contribution basis * Plan to do annual Roth conversions up to the top of the 24% bracket while husband is working Question If I pay conversion taxes from the brokerage, I’ll trigger capital gains and NIIT, making the effective tax cost +\~18.8%. If I instead use Roth contribution basis, the tax cost is +0%. I’ve always heard Roth should be the *last* account you touch, but in this case using brokerage dollars seems materially more expensive. Context * After my husband retires, we plan to fund living expenses from brokerage while harvesting gains in the 0% LTCG bracket until we start SS. * Remaining brokerage assets will likely be donated to charity Is it more tax-efficient to use Roth contribution dollars now to avoid capital gains + NIIT, even though that reduces Roth assets? Appreciate any perspectives or things I may be overlooking.