Hello!
I’m happy to hear you’ve taken a liking to growing your own potatoes! Coffee grounds were a good start to getting them up and going, but I think a simple layer of organic compost should do the trick. I would suggest testing the pH of your soil just to be sure that you are keeping it in a friendly range for your plants before fertilizing.
Potatoes shouldn’t require too much fertilizer after planting if a good base layer was set to plant in. Feel free to check out our pages on growing potatoes and where to buy our organic compost here:
https://www.planetnatural.com/organic-potatoes/
https://www.planetnatural.com/growing-potatoes/
https://www.planetnatural.com/product/organic-garden-compost/
While doing a little more research, I found a website with a gal who uses coffee grounds before planting and the progression she takes, along with another page that describes how to use coffee grounds in the garden with instructions.
Her experience: https://groundtoground.org/2010/12/10/potatoes-and-coffee-grounds/
Coffee grounds as fertilizer: https://groundtoground.org/2010/07/05/ground-to-ground/
Good luck!