I'm glad that article helped and gave you some ideas! And I totally get on the complicated therapy thing. If you ever need to discuss it (or vent about it) in the future, you're welcome to do so here.
Breast development is actually the only side effect of estrogen that won't reverse if you stopped HRT. If someone is doing HRT and then stops, their body reverts to how it was prior to HRT. As far as Viagra goes, according to the Mayo Clinic, some trans women on HRT do use it to try and counteract the erection issues that can arise, so taking it could make it easier for you to stay erect or orgasm. So it'd be an option to talk to your healthcare provider about as you adjust to HRT.
When it comes to anal play, this is a solid guide to locating and getting to know your prostate: https://www.vice.com/en_us/article/9k48 ... orgasm-sex
. Some toys are designed specifically for that, but it's usually easiest to start locating it with your fingers (since you can feel around with them better). Beyond that, anal sex works best with a lot of time, lube, and communication. Some people find certain toys or positions work best, some people find it works best if their partner just uses their hand. When you've been trying to find your prostate with toys in the past, have you noticed any angles or motions that tend to feel really good?