Despite having completed 11th and 12th with PCB, I'm passionate about exploring the frontiers of physics. What UG program should I opt for to become a professional physicist?

You can pursue your passion for Physics by enrolling in a Bachelor of Science program. Many reputable institutes across the country, including IISERs and IISc, offer BSc degrees in Physics. Admission to IISER is through the IISER Aptitude Test. You can read about admission to undergraduate programs at IISc here: https://bs-ug.iisc.ac.in/news.html. You should also check the websites for other programs. Good luck!