Keeping penis function (MTF, 19)

Hi everyone. I got an opportunity to start doing DIY estrogen through a friend. I’d like to keep my penis as same as possible. I enjoy topping and I’m terrified of losing that function, I don’t have any bottom dysphoria. What would be the best option for me? Options are topical testosterone gel, cialis or viagra. Also, how would I get any of those? My friend has a contact for a trans girl that is helping trans people get DIY HRT. I didn’t ask yet if she has access to any of it. What would be best option and correct dosage? I really need help figuring out what is the best option. Thank you.