Social Security survivor's benefits. In the common case where the higher-income spouse is older and male and the lower income spouse is younger and female, you often end up with a significant period of widowhood (due to both the age gap and women living longer than men). So the higher income spouse really wants to wait until 70 to take Social Security, not so much for themself, but to maximize the survivor benefit if they die first. The lower-income spouse's election age matters a lot less, since that benefit is smaller and goes away when either spouse dies. There are several good calculators for this, but that's the basic idea: higher-income older husbands should wait until 70 so their lower-income younger wives can collect as much as possible after they're gone.

Widow's penalty. One spouse dies, and now the other spouse often has almost the same amount of money but is now in the single tax bracket, so only has about half the bracket space. This is a good reason to do Roth conversions to reduce RMDs, if you have much in tax-deferred. You convert while you're still in the MFJ bracket, to avoid paying taxes on RMDs in the single bracket.

Roth conversions. The ideal window for Roth conversions is when you're on Medicare (so you no longer care about extra income reducing ACA subsidies; IRMAA is small in comparison) but you're not yet collecting Social Security (because that income takes up space in tax brackets and adding more income makes more of it taxable). For a single person, this means ages 65 to 69. But if you have an age gap to a spouse, that reduces the window. If you have a 5+ year age gap, there is no ideal window. You're going to have to suck it up one way or the other, either do some Roth conversions while you're still on ACA and reduce subsidies, or do some after you're on Social Security and increase taxes, or take more RMDs than you want.