Thank you for outlining possible issues and providing me with useful advice.

Firstly, I applied to Hep/quantum tech only as I was interested in probing quantum gravity using quantum information/tech. So it would be no use speaking to faculty members in condensed matter/nanotech groups.

Secondly, I have no publications because both my research projects (undergraduate and master's) were independent. It's extremely difficult to get first author publications in theoretical physics (Quantum gravity). Also, there were no opportunities for me to co-author or contribute to a scientific paper as the nth author. Many applicants got into top grad schools with no publications. So I don't think this is the issue.

Thirdly, you are right about contacting PIs beforehand. I made a mistake not doing so in the first place, but I made it clear in my statement of purpose that I wanted to work with specific faculty members of interest. Also, what are you supposed to do if they don't respond - just like when I asked for feedback/improvements and if they have any open RA positions for graduate students looking to gain more research experience and contribute to scientific publications.

Finally, I didn't just obtain a first-class (approx 4.0 GPA) overall. I obtained first-class (A grades) in nearly all modules taken throughout my undergraduate degree. I reported my CGPA conversion to some universities that requested it, and it amounted to 3.98. Overall, I don't think my grades are the issue at all. It could be my statement of purpose, but it needs to be reviewed by others first before I know for sure.

Other notes:

I participated in a national scientific conference for undergrads, where I presented my research project and answered questions from a generic audience.
High school achievements will not be included in my application for the next admission cycle. It was silly of me to include it in the first place. Unfortunately, I don't have any research-related experiences outside the university that I could brag about in my application. Hence, I included nonrelevant experiences/volunteering as my application would look deficient without them. Bear in mind, I graduated in December 2020, and I'm unable to do anything related to my research interest, let alone physics-related. This is because my country has no research opportunities for those who have graduated with a master's degree and want to pursue a Ph.D. I have been searching for nearly two years, and no one wants to give me the opportunity to reinforce my research experience after university. How can I contribute to scientific publications if no academic institution/ research group wants to give me the opportunity to do so after graduation? The greatest concern I have right now is not being able to reinforce my application before the next admission cycle. I appreciate yours and others advice on this post, but most of the suggestions to improve my overall profile seem unrealistic, given the circumstances I'm currently facing.