I started transitioning at the age of 40, and apparently I am already sometimes somewhat passing even though I haven't even gotten as far as starting HRT yet O.o

I don't think how long you have lived thinking you're male has any effect on how well you will or won't pass. (And also, the older you get, the easier it gets to pass anyway. Partially because looking good is less about having to show skin and more about having a good style.)

Based on what you're writing, you most likely will be much happier as an ugly woman than as a handsome man. And it's unlikely that you'll be as ugly as you think you would.

But most importantly: Forget about passing. Firstly, we tend to pass much better than we assume we do! And secondly, there are extremely bad-looking cis women who are absolutely women anyway, because being a woman is about being a woman, not about looking like a supermodel. Or even remotely good.

At least try it at home. Get a simple skirt from somewhere. And some makeup foundation. Use them at all peace within the safety of your home and boymode when outside your home. I know what will eventually happen when you use that strategy, of course: my last couple of months have been quite a ride!

Maybe be a girl at home, a guy elsewhere? Eventually you might start feeling that you'd prefer to present as female when outdoors, as well.

Also: Feminizing HRT won't really do much anything irreversible that matters. You most likely will end up infertile, but you can save some sperm beforehand, so whatever. In my case, I'm old enough that I don't really want to have any further children anyway. And then the only other thing is that if you wait very long before noticing that "no, HRT is not my thing after all!!!", you will have some kind of breasts that you then might want to have removed. That costs money, but not impossibly much. It can be done. So, all in all: Should you someday start HRT and then change your mind, you can just stop it and you'll be back to being male – the only difference being that now you know. You won't have to wonder what you should or should not have done.

It's bad to start HRT and then stop it and then start it and then stop it and then start it, etc. But if you start it once and then somehow stop it for good, then all's fine. Most likely you'll even regain your fertility, if a person of our age really even cares about that. Though, I'd say that once you get rolling with HRT, it's unlikely you'll ever want to go back to what you once were ;)