You're conflating a whole bunch of things. I'll try to be comprehensive, but I also don't want to write another book.
Because I know people that run the gamut of the the feminine masculinity scale. Men that are tiny, dainty effeminate and women that are anything but.
None of that supports the idea that gender dysphoria is biologically driven. Rather, it serves as a counter to that view - that there exist many small-statured males that seem effeminate but are still quite comfortable as males contradicts your assumption. That both Prince and Grace Jones exist as male and female respectively, without any demonstrable or known dysphoria suggests that there is a wide variety of body shapes within each sex, and that there is considerable overlap in characteristics that are correlated by not driven by sex.
I dated a girl for a short while when I was in my early twenties who probably had more testosterone coursing through her veins than me.
This is highly doubtful. Like, extraordinarily unlikely. The highest rates of testosterone production in females occur in females with PCOS, and that runs in the range of 2.0 to 2.5 nMol/L. On the other side, the rate of testosterone production in normal males is 10 to 35 nMol/L. Even in transgender identified males who are taking testosterone suppressants, the rate tends to be higher than 5 nMol/L. Levels lower than that are considered unhealthy for males, as there are fairly substantial side effects.
I know I felt and thought differently when I took steroids.
Everyone feels and thinks differently when they take steroids - they're well documented to increase aggression. I don't know where you're going with this... but "I took a drug and it affected my mood" isn't a particularly convincing argument for gender dysphoria being biologically driven.
I have a brother who is bipolar. This I'm told is the result of a chemical imbalance.
I have a sibling who is bipolar. Some of it is chemical, but not all of it. Most bipolar disorder has an aspect of heritability, and is also affected by neural connectivity and the communication between different parts of the brain. It's why drug treatment is only partially effective for those with bipolar disorder, and also why cognitive behavioral therapy is often very effective.
And again... that some other unrelated psychiatric disorder has a chemical component does not support your premise.
I see the way women act depending where they are in their cycle. I get moody some times almost for no reason. I don't know why.
Estrogen absolutely increases emotional lability. But the mood swings that females experience aren't caused by estrogen alone, they're driven by the change in estrogen and progesterone levels.
Is it your conjecture that transgender identified males have more estrogen in their systems than is normal for males? This isn't supported by any science that I know of. Even when taking exogenous estrogen, transgender identified males tend to have lower overall levels of estrogen than females do during the majority of our cycle, and they do not have varying levels - it's fairly steady.
I understand that while everyone is similar, we are also all different. Is it all learned behavior? Is it nature or nurture? Do either explain everything? I don't think so.
You might find it interesting to look into how paraphilias work. Most begin at certain developmental stages before puberty, instigated by external non-congenital events, making them environmentally triggered. What I find interesting is that they act like addiction, in that the more a paraphilia is fed, the stronger it becomes.
Why do I mention this, you might wonder? Well, because transvestitism is classed as a paraphilia, even in the most recent versions of DSM. And autogynephilia is a subset of transvestic paraphilias. And a rather large number of transgender identified males exhibit the markers of autogynephilia. Not all, certainly. But many.
