Has a doctor seen this? I know you've gotten tested for HIV and syphilis, but has a doctor actually seen it?
This sounds like balanitis. https://www.webmd.com/men/penis-disorder-balanitis It's not an STD.
Before trying to treat it yourself, you need to see a doctor and make sure that's what it is. Sometimes a bacteria causes it, so you may need an antibiotic.
Try to keep the area clean and dry, but definitely see a doctor soon. Let us know what happens.