Is it worth taking Fortnite on PC mouse and keyboard settings from professionals or rather not?

My Sensi was 800 DPI 1000 Hz and ingame 12 percent.

Currently 800 DPI 1000 Hz and in-game 14 percent.

Is it better to have fast sensi or low, and how do I find the best sensi for me?

You just have to try out what works best for you. Only if the professionals use a sensitivity of 500 DPI or whatever does not mean that you should use this sensitivity. You should just use the settings that you can handle best.

I would not change my sensitivity all the time, as it would be too much for me to get used to it.

The video explains how you can find good settings that suit you:

I think 400 dpi is the best. You are just super hard accurate.

unfortunately requires a large mouse pad. But you shouldn't go smaller than 400. But definitely use your arm to aim. Not just the hand. This also prevents illnesses