So I just received my atar estimate, which is 90.4 - is there still a possibility for me to significantly improve and reach atleast 95?
ATAR estimates aren't usually too accurate - at least the ones I got - until trial results were released. What that means is that there are a lot of opportunities to improve -
especially your trials, and of course the HSC. Do aim for as high as an internal mark (and rank) you can get though.
Keep pushing to boost your ranks, and that 95 ATAR will be a lot easier. One thing to note with Ethereum's point on ranks is the school you attend. A low rank in a good school doesn't necessarily confine you to a Band 5. That being said: work hard, grind for trials, and do those practice papers. Good luck.