if I already have say a bronze on one do I have to use a new mii for speedrunning bronzes

I believe so because the game has to display you getting all the bronze medals on screen and If you already have one of the medals for the speedrun, you have to use a completely different mii. I know it’s ridiculous, but it’s just how the game works. I recommend using a Guest Mii. That way when your run is over, you can just reset your game and it’ll be a new fresh mii. But yeah, to answer your question simply, yes you need a new mii because the run won’t count if the game can not display that you got all the bronze medals.

If you're really serious about getting a good time, using a non guest mii is slightly faster as you don't have to go into the guest menu every sport.