The usual idea is to have 5 years of expenses available in places that don't generate income when you use them to live on during the first five years of the Roth conversion ladder.
So a simplistic example using your numbers would be to have $200K in a taxable account in 2025. From 2025 through 2030, you would do two things:
1. Sell $40K of the taxable account to live on, and
2. Roth convert $40K for your living expenses five years later.
Starting in 2030, you would be able to harvest the $40K in Roth conversions from five years previous and could stop touching the taxable account.
With a $40K income and two people in your household and living in the continental US, you would get ACA subsidies up to 400% of FPL. For 2020 that number is $16,910 x 4 = $67,640. So the $40K number would put you at about 236% of the FPL, which would mean you could qualify for cost sharing subsidies at the CSR73 level and a silver plan.
The above glosses over some details:
1. You might have some capital gains embedded in that $200K in the taxable account. This will generate some taxable income which would affect AGI and ACA. But honestly it probably wouldn't be that much because you probably funded your taxable account after your tax deferred account. And if it is enough to matter, then just save some more to account for that.
2. Depending on how you're invested, you'd be taking somewhat of a risk that the $200K in taxable might shrink to $100K in 2027. How you address that risk is up to you. Some might invest the $200K in much more conservative investments. Some might be willing to go back to work, or pick up a side gig, or cut back spending.
3. The way I look at it, it is best to prime the pump on the Roth ladder after you retire - so in the example above you would do the first five years of your Roth ladder in the first five years of your retirement (2025-2030), not the last five years of your working career (2020-2025). This way you're not stacking your Roth conversion taxable income on top of your full time job income and paying higher marginal rates. However, this means waiting to FIRE until you have the $200K in taxable.
4. The $200K could be in other places - CD's, cash, checking, Roth contributions.
5. I'm doing some handwaving around 5 year periods. It turns out that Roth conversions done any time during the calendar year are treated as having been done on January 1st and they are available January 1st five years later. So a Roth conversion on December 29, 2020 is available for withdrawal on January 1, 2025 - a bit over four years later.