Seconding everything OP said and adding some [U.S.-specific] comments below the cut:
1.) As a trans gym bro, I need people to know that most gym bros are natty (meaning natural, as in not using anabolic steroids) and natty guys are very proud of being natty. Going up to a gym bro and asking him for illegal drugs is a dangerous and terrible idea if he's using, but if he's natty, there's like a 5% chance he's chill and takes it as a compliment on his physique and a 95% chance he's going to be incredibly offended and outraged. Getting yolked naturally requires consistent hard work over time, and implying that someone cheated to get the results they've spent at least five years working towards is near-guaranteed to piss them off. Do not do this.
2.) Illegal gym bro testosterone is dangerous. Some of it is black market and laced with other substances that purport to enhance muscle growth. Grey market stuff can be tampered with or improperly stored, either in ways that denature the medication or expose it to bacteria. I know a cis gym bro who got a very scary blood infection because he thought the 28-day disposal rule for multidose injection vials was just a suggestion, and he knew other guys who'd never had an issue with it so he assumed he'd be fine too. There are health risks that come with self-dosing and beginning new medications without regular monitoring or formal administration training, which can be mitigated through public trans educational resources and personal research efforts, but beyond that, the U.S. government can level serious criminal penalties against people who get testosterone without a prescription. Depending on what state laws are involved, illegally possessing a Schedule III controlled substance can constitute a misdemeanor, a felony with a required minimum prison sentence of a year, or a multi-year sentence with mandatory hard labor, potentially with steep fines, and almost certainly in a women's prison.
3.) The political climate is so hostile to trans people already. Trans people already get arrested at high rates and face elevated police brutality. A 2024 ACLU article cites recent LGBTQ+ survey data reporting that more than one quarter of trans respondents report experiencing police violence, with Black trans people reporting the most violence. The article also states that almost one third of trans respondents had been arrested. Law enforcement and politicians already look for reasons to hurt us, and freely admitting to felonies on the internet is not the move. I've heard reports of mail-order HRT being seized in some states, and seen an uptick in transphobic social buzz about DIY HRT. HRT Cafe shut down this spring after an attack, which is terrible for trans people in the UK. Multiple trans people I know who get prescription HRT shipped to them have recently had a package seized and were only able to avoid trouble because they could prove that it was prescribed. My friend got threatened by the police and, fortunately, he had his old prescription vial to show them. Can you imagine how, if that was grey or black market T, there might have been a different outcome? Can you think of any reasons why, when I see blogs encouraging transmascs seeking DIY T to DM them for help, my cop senses start tingling?
I understand DIY or die as a philosophy, and I understand that HRT is lifesaving healthcare. I know that trans people have done DIY throughout history, but there can be extremely serious repercussions to DIY T, especially the "just ask a gym bro" kind, that get glossed over on Tumblr to an alarming degree. I get that it's a last resort and there are a lot of scared, suicidal guys who don't know how long they can keep going without testosterone, but anyone considering DIY T should be aware of its risks and illegality so they can educate themselves and make an informed choice. That's not fearmongering. It is so fucking irresponsible to go "tee-hee, I love committing crimes and doing illegal testosterone :3" and act like that's the danger threshold for everyone, or say that it's not a big deal, really, and no bodybuilders ever get arrested for steroids, and it's not really illegal if nobody enforces it! If that's not a concern in your specific area and circumstance, good for you, but the U.S. is a big place and not everyone lives in an insulated pocket. DIY T is a high-risk activity that can result in you going to jail or being physically assaulted, and everyone who assumes that risk deserves to know exactly what the stakes are.