RMDs cannot be taken from a Roth. They are based on your ages and the balances of your tax deferred accounts, and taken from those accounts. Roth is not subject to RMDs.
I don't think making a huge conversion at a high tax rate is worth avoiding IRMAA for all but the one year but you can run the numbers and see.
Generally speaking, I would try to level out your income over the rest of your years, which means doing some conversions now, basically at the level of what your RMDs would be if you had to start this year.
To get more precise, I would lay out your numbers in a spreadsheet, and use
https://www.guidestone.org/resources/education/calculators/tax/tax1040 to determine your taxes at various levels of conversion, include the IRMAA impact.
Just my guess, but I'd look to convert to the top of the 24% bracket, and see how that looks. You are in 22% now without conversions or RMDs, and it doesn't take much to push you into 24%. You can either make a bunch of assumptions about future tax rates, return on investment, etc, or you can go with rough numbers that seem right.
It looks like you could convert about $200K to the top of 24%. That would get all of your tIRAs over to Roth in 4-5 years, depending on how much they grow before conversion. That would coincide with when the tax cuts expire and your tax rate (without RMDs or conversions) would become 25%.
So you convert now at 22 and 24% and then are at 25% later.
If you don't convert, you are at 22% now, but 28% later with RMDs.
If the lower tax rates remain, you convert now at 22 and 24%, then you're at 22% later.
If you don't convert, you're at 22% now and 24% later.
So to me that looks like you win if tax rates go higher, but it's basically a wash if tax rates stay the same.
I did not look at the IRMAA effects. I leave that to you.
