Later today we'll have CF Round #698, and one of the problem-setters is triple__a. I could not help but notice that he already has set problems for one contest, so I decided to give it a try and participated in a virtual contest. It was a Div. 2 Round # 630, but I still think you can make some useful observations. You can watch mine in the YT video.
Be careful with constraints, some of them are unnecessarily tight, in my opinion. Topics are standard: three implementation problems for div. 2 only, some number theory and combinatorics, dp on a tree, and later more advanced stuff. There was also an unusual problem about finding the bug in the given code.
The modint class helps a lot, and you should have it. A good sleep schedule is crucial. Do not write contests straight after you wake up after a short sleep, or at least be very careful. Finally, you can save a lot of time if you just keep solving problems instead of complaining about them :upside_down_face: