It's already very easy to have a marginal rate around 30% at relatively low incomes when you're on ACA health care with premium tax credits. Income around 3x the poverty level can often do it. Once you age into Medicare and don't worry about the ACA phaseouts anymore, a single senior would need a gross income over $208.5k to be in the 32% bracket, while a married couple would need about twice that amount.
If you are living primarily on long-term capital gains (as in OP's case) your Federal taxes are 0% + ACA phase out. Much less than 30%.
You still need to watch the development, and the cliffs, when and if they come back. And, make sure you don't run out of taxable account before you intend to.
The OP expects their taxable account to be enough that a Roth conversion ladder will not be necessary to pay their bills during retirement. They have a solid plan for how they will pay their bills from side hustle income, taxable brokerage interest and dividends, plus some capital gains as needed. The main thing they're looking at is whether it makes sense to do some Roth conversions anyway, on top of their existing planned income, in order to reduce RMDs later.
Their side hustle income is expected to amount to about half of the married standard deduction. They also mentioned having some of their taxable brokerage allocated to cash (which would yield interest taxed at regular rates). They might have some other investments (REITs, bonds, etc.) that would also generate income not subject to favorable qualified dividend rates. Including the long-term capital gains they would need to realize to pay their bills, they aim to keep their gross a bit below 200% of the FPL—
$74,580$49,720 for a family of three. The amount of space they have left for regular income in the 0% standard deduction zone is perhaps $10k. That amount of conversion might be worth looking into. Beyond that...they'd be paying 10-12% of the next $94k of conversion in federal income tax, the premium tax credit phase-out would add 10-18% more, and once the conversions go beyond about
$50k$65k they start pushing some of their 0% capital gains/dividend income into the 15% bracket. The cost sharing reduction cliff at 200% of the FPL is worth real money as well. Add it all up and you're locking in a minimum 20% marginal rate on any meaningful amount of Roth conversion, and it could easily meet or exceed 30% overall.
Now, until the end of the year their kid turns 16, they do have a kid who will be eligible for the child tax credit. Part of that is non-refundable, so for the time being they may have a bit more room to do some Roth conversions without adding to their federal income tax. Only the lost CSR and premium tax credits would be a factor in that range. So for the next several years a Roth conversion in the range of perhaps $10-30k might come in at a reasonable rate. They'd have to run the numbers. The year the kid turns 17 the range of 0% marginal federal tax narrows. A few years later when the kid stops being their tax dependent, their family poverty level goes down so that changes the math a bit on the ACA phaseouts.
I still think the window where both members of the couple are in the 65-75 range is likely the optimal time to do some bigger Roth conversions. They might be able to do some smaller ones at reasonable rates while on ACA insurance, but nothing that really moves the needle.
This stuff is complicated!
edit: multiplied wrong on the 200% of FPL.