I'll touch on a few things:

Gonna be honest, I’ll start this off with saying I can’t tell if it’s normal for other trans people to have to internally transition as well? I still don’t have an internal sense of anything, but quite often use my birthname or use masc terms in my head (without much thought) before catching myself, and I’m not sure if this is a normal thing?

Yes. There is an internal transition process. And it will continue for years. You've been using and responding to your birth name for years. That's not going to wear off overnight. When you transition, you really go through a process of breaking down and rebuilding your sense of self. You're not a completely different person on the other side, but you're also never the same either. Your way of interacting with others will subtly shift. I mean, hell, if you start being read as female by strangers, that alone will affect how you move through the world. Suddenly all the social carrots and sticks that get applied to cis women get applied to you. And just like cis women get conditioned by it to mold their behavior, yours is affected to some extent as well. That doesn't mean you're going to become a 1950s housewife, but no one is immune to social conditioning. Then add on the whole internal transformation that comes from asking, "ok, I'm a woman. What kind of woman do I want to actually be?"

Are homebrew injections a good idea?

Yes, especially if you're in the UK. I have no idea how long it would to take to get an actual prescription on the NHS, but it's infamous even over here among trans people in the US. And the thing is, there are enough people on homebrew HRT at this point that if there were some serious risks from it, we would already have an epidemic of injection-site abscesses and infections. Yet that hasn't happened. There's enough people on this stuff that if it were that risky, we would be hearing about it on the news.

The reason for that ultimately is that hormones, as chemical compounds, are pretty heat resistant. This allows home brewers to prepare sealed vials and then place the entire sealed vial into an autoclave. (Something like this.

This is what ultimately makes home brewing viable. They use filtering and sterile techniques such as still air boxes and laminar flow hoods, but ultimately it's the autoclave that does the heavy lifting. And because E and T can take the heat, brewers can make a vial in the imperfect conditions achievable in a home lab, seal it, and the heat kills any bacteria that might get through the imperfect preparation process.

And not all medications can do this. Some simply can't survive much heating, so if you tried to homebrew them, they wouldn't survive the sterilization process. They can't be heated hot enough to kill any bacteria in the solution without also breaking down the medication itself. There's a reason homebrew insulin is not a thing. Insulin, as a molecule, simply can't take the heat!

If so would starting on monotherapy (and measuring my T-levels) be a good idea?

It works for most people. But I suppose it's what you want to do and what your budget looks like. If you can afford it, purchasing an anti-androgen as well isn't going to hurt and might help. But plenty of folks do just E. When you take a high enough E dose, your brain detects that the body already has sufficient sex hormones, so it shuts off sex hormone production, nuking your T production. But you should definitely verify you levels via blood work if at all possible.

Breast shape

There isn't really any evidence for this. There is now actual scientific evidence of progesterone helping breast growth, but effect of P or E dosage/delivery method is completely anecdotal. Just focus on getting your T suppressed and your E up, and hope for the best. That's really all you can do. Yes, there is a limited argument that in cis girls, they start growing breasts with lower E levels in early puberty. But reality check, unless you're that age, we can't assume that would transfer over. Maybe it would be applicable to someone who gets on E at an extremely young age. But for adults? Your growth hormone levels are completely different. That alone will mean what's applicable to cis tweens cannot be reliably predicted to work on trans adults.

Storage

Remember, hormones can take the heat. The DIY vial you got survived the conditions inside an autoclave. (Temperatures beyond 100C, a few atmospheres of pressure.) Now, duration as well as temperature do affect durability. But unless you're trying to store vials for decades, a few months at 25-35C is fine. Really the big thing is to keep them out of ultraviolet light. That's what will really degrade them. So don't store them on the windowsill or somewhere sunlight can touch them. Humidity is irrelevant, the vials are sealed.

Should I spend the extra effort to find my minimum required dosage to suppress testosterone (in monotherapy)? I get you should be aiming for it but since injections could be every 1 to 2 weeks and I need to wait for it to build up in my body first, if I can’t get blood tests done for free I might just have to settle on what works

Really, don't bother. I like my E on the high range. If my estradiol is anywhere from 300 to 600 pg/mL I'm happy. The thing to understand is the human body has the ability to safely handle very high estrogen levels. During pregnancy, estradiol levels climb up into the 3000 to 6000 range, and stay there for months. Now, trans guys do have to worry about taking too much T, as excess T gets converted into E. But for trans women? There's little downside to E levels a little on the high side. Find a range that makes you feel good and gets you the results you want, don't go too crazy, and you'll be fine.

But like, for reference, I'm a bit of a mad scientist. I once wanted to see what pregnancy-levels of E felt like, so I took 40 mg (1 mL of 40 mg/mL EEn) for several months. My "high score."

🤣🤣🤣 🤣🤣🤣

Got some interesting effects, but had zero impact on my health. I don't recommend you try that, it was a risk and I willingly took it. But it shows that you really don't need to worry too much about getting your dose a bit high. You could probably take 10x your ideal dose and have no negative effects except for some really strong emotions. Though being on that kind of crazy dose for long term could probably cause some weird effects. So you don't need to stress too much about your dose being too high.