IMO for the same reasons you should follow the
recommended investment order when contributing, you should reverse it when withdrawing. That would mean you should spend down taxable first.
For us, we have a ridiculous amount of tax advantaged space, so even having enough available in taxable and roth contributions to get a roth conversion ladder going is going to be a challenge. For those with a larger proportion in taxable as you describe it might not make sense to live off money from a roth conversion ladder, but rather live of money from taxable for as long as it lasts.
Whether or not you live off money from a roth conversion ladder, however, you should still convert enough from traditional to roth to fill the lower tax brackets. Definitely the fill the standard deduction, probably fill the 10% bracket, and maybe fill the 15% bracket. Fill out the rest of the 15% bracket with 0% taxed capital gains. All of this while keeping an eye on health insurance subsidy limits and tax credits of course. No sense in "wasting" the lower tax brackets just because you don't need to withdraw the roth conversions to live on. Conversion amount and spending amount don't necessarily need to be the same.
And correct, roth balances are not included in RMD calculations.