Hello! I'm a sex ed teacher who works for an lgbt org (and also happen to be t4t) so might be able to help with some of your questions!
I would recommend PREP if you're having any sort of sexual contact with someone who's HIV status you don't know, or someone you know to be HIV+ and not undetectable. (No pressure obviously! But it's a really cool thing to exist, and if you can get access to it it'll take a weight of your mind and also almost eliminates your chances of getting HIV).
HIV is only spread through blood (that's why it's most commonly talked about in relation to anal sex - because there's a higher chance of blood). But keep an eye out for rag nails or tears around your fingernails (that's enough blood!), and cut gums from brushing your teeth (you should preferably wait 3 hours after brushing your teeth to have oral sex, and wait until 3 hours after oral to brush your teeth again).
Being t4t also brings a few additional risk factors, unfortunately. It's common for folks on testosterone to have thinner vaginal walls and make less natural lubrication due to vaginal atrophy, which raises the chances of rips or tears during sex. Similarly, people who have had a vaginoplasty often won't lubricate to the same level someone with a pre-built-in vagina would. It's also common for people with penises who are on estrogen to have thinner genital skin, which can also tear easier.
PREP doesn't affect hormones at all, so no worries about it interacting badly with HRT. I have heard a few people have an upset tummy for the first day or two on it. I personally had no issues, but maybe best to stay near a toilet after your first dose to be safe!
When taking PREP you can take it every day, or just for 7 days before and after vaginal sex, and 3 days before and after anal sex, and (like with birth control pills) make sure you take it at the same time every day, and bear in mind that if you vomit or have diarrhoea afterwards it won't be as effective.
For getting tested, I would recommend checking if your local hospital has a GUM (genitourinary medicine) clinic, and what their policy for trans people is. Here our GUM clinics are gendered but trans people can use whichever is more comfortable for them. I generally prefer going to GUM than my GP, because then I know they're experts, and also they spend all day talking about sex so there's less embarrassment. I'd also recommend reaching out to any local LGBT orgs and seeing if they do testing or have GUM outreach clinics.
As for birth control there are a few options that won't interfere with HRT, but how available they are might differ from country to country. Unfortunately atm there's only condoms for amab penis havers. For afab folks there's a myth that t will make you infertile, but unfortunately there's been no reliable research into that. I've heard of people get pregnant while on t, and also of people who go off t to get pregnant intentionally and have a lot of trouble, so it's probably safest to have some back-up birth control just in case. Condoms, internal condoms, caps, diaphragms, and IUDs are all definitely compatible with testosterone. I've heard that some forms of hormonal birth control (particularly the ones that only contain progesterone but not estrogen) are compatible with some forms/ brands of t, but talk to your endocrinologist if you have one (or whoever monitors your bloods) before getting started to make sure.
If you really want to be as safe as possible from all STIs you could also have a think about using condoms and/or dental dams for oral sex and rimming, and gloves or finger cots for fingering. If you're giving oral sex to someone with a penis who isn't able to stay hard enough to wear a condom you can also use a dental dam for that, as long as it covers all of the skin you're touching with your mouth.
I hope some of that helps!! I also made a zine for a trans masc sex worker friend I'd be happy to send you/ anyone else, but some of the info in there might be UK specific.
