Not enough information here to be too specific, but if you are very concerned with RMDs, you have to pick a tax bracket you are comfortable with paying today to mitigate the risk. Lets say you have $50,000 in other taxable income (on top of any deductions), and you are OK with paying 15% taxes today. The 15% bracket caps out at $73,800, so you would convert $23,800 to Roth for this year. Do this type of thing each year until the accounts in question are fully converted, or you decide to change strategies, or the RMDs kick in. You can continue convert amounts over the RMD once they start, but the RMD itself cannot be converted.
If you are OK with paying 25% taxes today to mitigate the risk of the RMDs becoming a problem, then you can convert until your income is up to $148,850. I personally wouldn't do that, but if you think you are likely to be paying a lot more than 25% taxes on this money down the road, then maybe it is worth it to you to pay that much in taxes today.
I also think you might be worrying about this unnecessarily - if you had to take an RMD today on your ~410,000 in retirement accounts, even if you were already 92 years old, the RMD would be less than $41,000 - you probably would need at least some of this amount to supplement your SS income anyway, and it wouldn't, by itself, push you past the 25% bracket (or even into the 25% bracket). And if you really don't need a particular RMD in a given year, you can donate it to charity if your goal is to avoid taxes. Or pay the taxes and buy some more funds in your taxable account if you want to leave more to heirs.
I guess my point is, the RMDs aren't as big a deal as some fear. If you were going to have many millions of dollars in 401Ks it might become an issue, but at more typical levels the dollars and cents aren't that big of a deal, even if you get hit . If you really don't want to make them for convenience sake, or because you think your taxes are going to go up so much that the math will work out, you have to draw a line in the sand of how much tax you are willing to pay today, and convert up to that line but no further. For me, I might consider going up to the top of the 15% tax bracket, but certainly would not be converting in a higher bracket. You've just got to decide where that line is for you, and proceed accordingly.