There is no 15% tax bracket for ordinary income (e.g. a pension.) There is a 12% bracket.
https://www.gocurrycracker.com/tax-reform-early-retiree/The answer to Roth conversions or capital gain harvesting could be neither.
If you get health insurance via the ACA, that is probably the case unless you have a really big TSP.
For Roth conversions, if you can pay a lower tax rate now than you will at age 72+ with RMDs, then do them. If not, don't.
If you also have the opportunity to harvest capital gains and pay 0 tax and 0 increase in health insurance premiums, then also do those. If not, don't.