I’d say shut off one and do the peroxide if you can. If it’s primarily an anode problem, the odor will come back within a week or two. If it’s stagnation, it won’t.
Solutions: they’re all messy, I’m afraid. Larry may have something to add, but one solution here would be to replumb the heaters in series. If it’s the anodes, then you’d need to change to powered ones. They never cause odor.
I hate it when that happens, but 99 percent of the time aluminum/zinc solves odor problems in unsoftened water and they are far cheaper than powered anodes.