Give me a strategy for getting to 500% multiplier and/or donut farming?

I think it depends on where you are in the game. My multiplier is just over 730%. I do have all the character always available as well as all of the buildings and most of the unique decorations. I am able to buy all of the new things as they come available. There are only 3 characters I don't have so I usually don't have to worry too much about the old content that comes back. I will increase my multiplier as decorations that I want come available but I am able to do what I need to with where it is now.