"We have discussed the advisability of future IRA Conversions in the years after he retires."

At that point, it's hardly worth it. Paying tax on conversion or paying tax on distribution is the same, and there isn't as much time for tax free growth, once you are over 70, unfortunately. It's a bit of a gamble if you will live long enough for that conversion to ROI, and now it significantly impacts IRMAA. So, there is that additional cost on top of the taxes, which would not be a cost under taxable RMD.

I typically advise early retirees (non-working) to do all planned conversions by 66-68 and for workers to do planned conversions by only a year or two later. If IRMAA is a consideration or they need to collect SS at 62, then also do conversions by 62.

But, if they are sitting pretty, then later conversions (although taxable) when they won't touch a Roth, still are nice for the beneficiaries, as long as the 5-year rule is met. And, again, since you most likely don't know when you will die, it pays to make conversions sooner rather than putting them off until later.