He said that grease was too thick for kart bearings.
kind of a blanket statement that isn’t universally true. lot of misconceptions about grease in bearings. Google around and find an article about how the grease lubricates the bearing. It is more dependent the balance of heat and “melting point” of the grease (which increases with the “thickness”)
you are on the right track – find a lighter grease that is better suited for the temerature profile of your bearing.