The problem isnt in the top layer, you have gotten below the top of the laminate. There is not a lot you can do to fix them. You could replace them, if you had some in an out of the way place. It would be a job for a professional. Laminate isnt a wood. Laminate has almost a paper type top with woodgrain imprinted in it. Once you have damaged that (which you did when sanding), there isnt anything you can really do other than replacement. Here is a link that will explain how laminate is made.

https://www.swisskrono.com/us-en/about-us/swiss-krono-usa/the-learning-center/planning/articles/what-is-laminate-flooring-and-how-is-it-made/#/

Bring in a pro, pay them $50/hour (with a minimum job charge of 2-4 hours) and get them to pop those planks out and drop in new ones....that's assuming you have EXTRA planks????