What are the best ways to grind Wheat Slims?
As idle games go, the more you play, the more you gain. If you keep the game open, not only do you have the opportunity to collect Golden Cookies, you also collect your usual amount faster than if the game was closed.
If you get your farm up to level 9, you will be able to plant up to 36 Bakers Wheat at once, which will drastically increase your odds of obtaining Wheat Slims, as they only have a .1% chance of dropping when you harvest a mature Wheat.
As for the rest of your garden, I would recommend checking out the Wiki for plant combinations for new seeds.