Use text alignment to center or right-align items in the hamburger drop-down menu. Flex settings are breakpoint specific. Expand the the triangle to see the different breakpoints and adjust accordingly. What you are doing is very bizarre and not typical UI. When a menu has too many items to display on one line, then change the breakpoint at which it switches to mobile. Reduce the padding between the links. Use a narrower font or reduce the font size.
Experiment! Try stuff out! You have to learn the program. Can’t keep coming here every time you run into a basic feature you don’t understand. Watch the videos. Read the tutorials. Build a site alongside one of the videos. Read the Bootstrap tutorials on W3Schools or the Bootstrap website. This forum category is for help with using the software, not teach people step-by-step how to customize different components.