Is saying "May Allah reward us and you with the Highest Paradise" in response to "May Allah reward you with goodness" considered an innovation, given that this is done based on the noble verse "And when you are greeted with a greeting, greet in return with one better than it or [at least] return it [in a like manner]"?

It is permissible to respond with what was mentioned, and it is not considered an innovation (bid'ah). Firdaus is the highest level of Paradise, and a Muslim should ask for it, as the Prophet (peace be upon him) said: "When you ask Allah, ask Him for Firdaus." The meaning of "greeting" (التحية) in the noble verse is peace or supplication.