I recently used enough Banana Farmers to unlock their Pro variant, and I realized I do not like it. After running into some issues with it, is there a way to purchase normal Banana Farmers, instead of the Pro variant?
Unfortunately, no. Once you unlock the Pro Agents, they will remain the Pro variant for as long as you don't reset your progress.
The purpose of the Pro Agents is to provide a more useful alternative to each agent as a sort of reward for purchasing 50 of the same agent collectively. I guess if the Pro Farmer is causing you problems the only option is to keep the farmer out of reach from the track or close to the exit if possible.
No, you cannot buy normal agents after having them as pro agents.