Well that gets real complicated figuring each of their tax rates in the future.
And who knows how they are going to change the tax laws in the future.
Yep, you are right about that!
BTW, I am far from dogmatic on whether (and how much) one should convert. As you say, there are so many factors that complicate the decision, and some pertain to one person and not to another. I am only trying to convey the rules of thumb and try to encourage how to make the comparison fairly.
In my case, one of the main drivers for converting is the assumption that my DW will be filing single at some point, and so her future tax rate will be higher even if tax rates don't change. The other main consideration is that I can use pre-medicare and pre-SS years to convert, and thereby keep us out of higher IRMAA tiers if we convert, and not if we don't. But, I do want to preserve a sizable deferred bucket for possible QCD and/or medical/LTC expenditures.
On the other hand, I have almost no bequest motive, so that eliminates a host of considerations that you (homestead) have to contend with. For example, pb4uski's recent comment about preserving taxable for inheritance (in consideration of the stepped-up basis) was not one that I have had to contemplate.
If I do multiple smaller conversions the end total is 116k larger and 144k total taxes.
Less taxes now and more later on.
Ending up with
55% aftertax
25% roth
20% ira
If I do multiple conversions (3x above amount above) total taxes are 106k
More taxes now and less later on.
Ending up with
35% aftertax
55% roth
10% ira
You
almost gave us enough info to figure out your situation.
(Actually, I thought you did, but the numbers do not appear to be all internally consistent, maybe?) Not that you asked, but I personally would be hard-pressed to choose between those situations, but then I don't have all the information.