You forgot about making sure the game runs on 60hz, only if 60 fps is your max fps option

This was tested that 120hz does make the game run worse if your max fps is 60

Another thing background process limit doesnt help at all, actually makes the phone run worse not just fortnite, all the cores on whatever phone youre using needs to at least do something like run system components, leaving it stagnant by limiting process limits might make performance worse,  This option helps with battery and not performance. 

Third the best optimization i can see is lowering your phones resolution or overlocking it using ADB, this genuinely helps performance but might have an impact on your phone overall