I am preparing for the physics competition now. The following is my personal experience:

1 Start with calculus, you don't need to learn very deeply, just learn some fur. Many good physics textbooks use this, which is very helpful for you to read these textbooks! Moreover, studying calculus is also very helpful for you to understand infinitesimal methods in physics in the future.

There are not many supplementary books. Personally, I think it is enough to have two books on mechanics and electromagnetism by Cheng Jiafu. If you are interested, you can buy another standard Olympic Games textbook compiled by Zheng Yongling. Both books are comprehensive, and the scope of the competition is listed at the end of the book. After reading these books in a down-to-earth manner, basically it should be no problem to save the first prize.

For extra-curricular remedial classes, it is recommended to report when you have a little foundation, and then listen after you have more systematic knowledge, which is better.

It should be no problem to buy some books for pure practice before the game and practice for more than a month to take the exam.

There were many detours in the physics competition before. The above is my personal experience. Finally, I want to say one more thing. Since you have chosen the physics competition, don't give up because you are frustrated halfway. Maybe if you stick to it for a while, you will make a qualitative leap. I hope what I said can help you, come on!