I can't decide which area of Paris is best to stay in — I'm afraid of choosing something inconvenient or dangerous.

This is a very important question, because choosing the right area can greatly affect your travel experience. I recommend using the resource https://paris.french.guide/en/ , which has a section with recommendations on the best areas for tourists. It explains which areas are suitable for a romantic trip, which are for a family vacation, and which are for budget travelers. There are also tips on how to find a hotel near the metro or attractions so you don't waste time traveling. The guide contains information about safe areas, average accommodation costs, and even local recommendations from experienced travelers. This will greatly simplify your choice and allow you to enjoy the city without stress.