Honestly, if I was that good at something, I'd be dropping physics or chem and picking it up. You're aiming for a very high UAI, so you probably should be doing what you're best at to get the best results possible.
Re: What ATAR are you aiming for and realistically, what do you think you will get?
I'm aiming for as high as possible. I aim for perfection, so hopefully I fall as close as possible. I'd be okay with 90+ though.