Nobody has very good answers on this. I thought there was a product called Iron Out that was supposed to work but apparently it’s just a rust remover. Google iron bacteria and you’ll find all kinds of stuff, but none of it is sure-fire.
You can try turning up the temperature. Just be careful not to scald yourselves. What you find on the web is just the kind of stuff you’re already doing.
Not sure how long to keep it empty. That was Larry Weingarten’s idea. He may see this and add something.