@seshu_ram yes exactly. Even I want a pure science course. And of course, after IISER the best place to be is IISc. There are pure science courses in IIT (B.S chemistry, the one I'm interested in), but since IIT and BITS are "technological" institutes, I doubt how effective it will be. I have friends in IISc and IISER, and they talk about doing hours and hours of practicals, and even being left alone in open botanical gardens to conduct their own research. That, I believe is the essence of studying science.
Whether this happens in IIT, I don't know, because what I have heard is people only take up BSc in IIT to change their branch in the next year.
Are you still considering KVPY SX stream? You can do it, bro. You're bright. Try for KVPY, and go to IISc. If that does not work out, and you're still interested in science, second choice should be IISER. Lastly, considering the unpredictability of JEE Adv, keep MSc in BITS as plan C.
Oh yeah, and keep preparing for JEE Adv like you're doing, and when you get a good rank, you don't even have to write the IISER aptitude test. You'll absolutely get IISER Pune.
Good luck, and Don't hesitate to ask if you need anything else.