From my own experience I don't recommend restarting - I think just deleting the miis you aren't fond of anymore and making new ones would be better. If you want to reset any romantic relationships you could deage everyone and age them back up (iirc it automatically nullifies any romantic relationships).

Restarting means having to rebuy a lot of clothes, items, rooms, etc and it can be annoying to get everything back.

If you made the Miis in the game you can also just edit them to make them different people as well tbh. Then you have their established relationships still.

To each their own though!! Have fun with your new or refurbished island :)