Keeping penis function (19, MtF)

Hi everyone! I got an opportunity to start DIY estrogen through friend this year. My friend has a contact for a trans girl dealer that’s helping trans people get on estrogen here in Slovakia.

I’d like to keep my penis functional as I enjoy topping and I don’t have bottom dysphoria. I did a bit of research and options are: topical testosterone gel, cialis and viagra. Which one is the best? How would I get access to each of those (I don’t know if the dealer has any access to these)? What would be correct dosage for all of these? Thank you for help!