Hi So Close
I've just been thinking through this myself. Have you seen this post yet? (Warning, it is really messy and complicated)
http://www.gocurrycracker.com/obamacare-optimization-vs-tax-minimization/tldr: 25% marginal rate on all Roth conversions above 138% FPL, and up to 15% "tax" rate on dividends and long term capital gains as a result of ACA subsidy reductions, less if you do large scale conversion
Assuming 25 years worth of assets, 12.5 in taxable accounts with 20% gains, you are looking at maybe 2 years worth of spending in gains. Trying to harvest all of that quickly would certainly require paying taxes. The 0% space isn't going to be large enough, and if you harvest up to the top of the 15% tax bracket you wold have no ACA subsidies
I've also always prioritized Roth conversions in the 0% space since RMDs will bring tax rates of 25-39.6%, whereas the tax on gains is max 15%
If you spread the gain harvesting over n years so as to still be paying 0% (~$20k/year for MFJ), then your Roth conversions will lag. Projecting 401k/TIRA account value forward to Age 70.5 will provide some guidance about relative importance (roughly having $500k in 401k/TIRA at age 40 for MFJ is the max for being able to pay zero tax on Roth conversions pre-ACA)
All of that said, if you are young and healthy, choose an ACA bronze plan for minimal/negative subsidy cliff, you might be able to pay something as low as 5-7% average tax on large gain harvesting in one year (see chart/table in that post.)
Or you could go abroad for a year and pay 0% on the gain harvest. Uncle Sam would help with your travel bills in that case