It's extremely effective after a long enough time, because it will eventually shut down your ovulation.
Unfortunately, nobody knows exactly how long "long enough" is, it probably varies from person to person, and it's unlikely we'll ever have a definitive answer because the studies required to find out would be extraordinarily irresponsible (you'd have to have a quite large sample of gay trans men having vaginal intercourse using T as their only form of birth control, with the knowledge that any resulting pregnancies would be at exceptionally high risk for severe birth defects and would also put the mental health of the subjects at risk).
Any responsible doctor will tell you that you should always use a barrier method of birth control while on T, until and unless you have a hysto and/or oophorectomy. It's an open secret that guys in committed relationships who have been on T for years don't usually use birth control, but no medical professional would sanction it.
Edit: That being said, I jsut reread your post. You say you're anal only and a top. Topping puts you at zero pregnancy risk unless you are rubbing your genitals in your partner's jizz. Same for oral. If you do bottom, you should never have unprotected sex anyway unless you are in a committed monogamous relationship and both of you have seen each other's recent negative STD tests (standard gay safe sex lecture: STD rates, especially HIV, are dangerously high in the gay community, and bottoming for anal sex carries a higher risk of STD transmission than anything else you can do in bed). If you are bottoming for unprotected anal sex in a committed relationship, it's a very low pregnancy-risk activity; the only risk is if semen contacts the vulva (either directly or through after-sex leakage), so avoid letting your partner cum on or near your genitals, and shower and clean up after sex especially if he cums inside you.
