I learned late in the game that the secret to large amounts of free donuts is to build a high multiplier rate. Then when you farm donuts (by buying and selling rat trap trucks), you must have the collider running during the collection to get the most xp. So at first I looked for items with the best ROI, which is about 10 donuts for 1% increase. (I prefer jetbikes and checker-striped bombs) I would run the collider only during the donut farming sessions.
You will want to farm with KeMs first, and you'll want to get your rate above 300% to really see an improvement in donut earning.
Now that my rate is 4000%, I rarely ever farm; but I keep the collider going permanently, because the one time daily rent collection with the Skyfinger monument now generates enough pink sprinklies to get by on.
