After a run is there a way for me to easily reset top the beginning of the game? Or do I have to make a new Mii every time?

to be able to redo the race I have a method which consists of going to the console settings and going to the files (dowload data ...) and going to the splatoon file then deleting your backup with the mii especially not something else

otherwise I advise you to go to the splatoon discord because we will deal with your question better.