The ACA premium tax credit is, as you point out, on a sliding scale from about 3% to about 9% of AGI between 100% and 400% of FPL.
Being FIREd, within broad limits I get to decide how much income to realize each year, mostly through Roth conversions and LTCG. I view myself as having four distinct taxes on AGI: federal income tax, state income tax, loss of premium tax credit, and FAFSA EFC. When deciding how much income to realize, I look for significant "breakpoints" and the overall marginal tax rate. So for example, I may be paying 10% federal, 7% state, 5% ACA PTC, and 5% FAFSA, for a total of 27% tax.
There are three reasons I can think of to do tax gain harvesting in FIRE:
1. To lock in the tax rate on the gain at current rates. Some people think tax rates will go up in the future, either because of US government tax law changes or because their personal income overall is going to go up. So paying 0% LTCG now (assuming a modest FIRE income) is preferable to paying 15% LTCG later.
2. To absorb non-refundable tax credits. There are a few non-refundable tax credits which are wasted if there is no tax to offset. The ACA PTC and federal taxes can be offset by these credits.
3. As a more specific case of #1, there is something called the tax torpedo. Basically at age 70, a person who has FIREd will have Social Security and RMDs which will put them into a fairly high tax bracket. Realizing taxable income (through either Roth conversions or LTCG) now at a total of 27% and getting those monies into an essentially tax free state may be better than paying 45% or more later.