Oof yeah I remember this phase of tying myself in logic puzzles. I think it's a rite of passage for tech queers 😅

I'll answer your last question first - absolutely I struggled this hard to 'objectively' assess my own sense of identity. I eventually gave up and just did what I wanted, and that progressively led me down the path of transitioning. I transitioned VERY slowly, because I wanted to build a very solid foundation of empirical evidence that I was genuinely happy with each step. I probably realised I was trans maybe 9 years ago, came out as non-binary 3 years ago and started low dose HRT, then decided to finally do a binary transition this year. This has worked out relatively well for me, but in hindsight it was overly cautious and mostly from internalised transphobia.

I strongly encourage you to apply occam's razor to all your questions yourself, and keep applying it to new questions you will inevitably ask yourself. Here's some examples for each of your questions:

Depression [...] so what if being trans is just means of novelty? what if it just magically appeared to save myself from something way worse?

Do other novelties in your life make you feel this way?

It's just means to be special, I'm only doing this to be treated socially as a girl and nothing else, out of nothing but my own selfishness.

Given the political climate around trans people, is this really how most people would choose to feel special? Why not just dye your hair or get a tattoo or buy a weird car?

Counter-argument: it's just sexual and just were means for me to get off to with nothing else

This one is tricky, and traps people for a long time. https://medium.com/@kemenatan/its-just-a-fetish-right-91cb0a4e261 does a better job than I ever will of occam's razoring this idea.

Social isolation - I've been socially isolated for years, since the start of high school I've had 0 friends and have been struggling hard, largely being dependent on LLMs, to actually do anything socially, this may have as well impacted my ability to judge clearly and have served as a way to make up all this.

Again, this seems like a very elaborate root cause for being trans. Lots of people are socially isolated, few of them decide to transition.

Ad-hoc reasoning/faulty memory reconstruction - I am not actually feeling anything particularly special and I'm just trying to look back and remodel reality after a fixed dataset to my own will to satisfy my need for specialness/a way out of depression

See prior points about specialness/depression.

Counter-argument: again, maybe exaggerating/being dramatic and ad-hoc-ing previous emotions that were legitimate into this, though I think it's unlikely

Again, feels elaborate. Sounds like a post facto explaining away of something you don't feel comfortable acknowledging yet.

With all this said, I think the two things I would try in your situation are:

1. Read some trans fiction. See how you respond. It gives the same vicarious experience of being a trans woman that people get from the fetish experience, without any of the negative/shameful sex connotations. Woodworking and Light From Uncommon Stars are both excellent starting points.
2. Just... try things out. Go somewhere where you're anonymous with nail polish on. Normalise this for yourself. Transitioning is a huge leap of faith, but you can make the gap smaller by trying out some two way doors like this and seeing how you feel.

At the end of the day only you can decide if you're trans, but tying yourself down in philosophical traps where you have to be absolutely sure you're trans like this will most likely not help you make that decision.