My Mom does not bike. She was having issues kinda similar to this on her behind.
She had bought laundry detergent pods at a marvelous discount. It turned out that her washing machine needed an extra rinse cycle for underwear with the dose of detergent in the pods.

There is 1 guy in my Tuesday night ride group who gets a chest rash from some jerseys, and has to wear one of those wicking exercise sleveless undershirts now under a bike jersey. Perhaps one of these might be worth a try for you, to see if that plus no HRM strap makes any difference.