need help installing bathroom solid surface countertop that does not sit flush on vanity
In my limited experience, the top is commonly adhered with either silicone caulk, or hot glue, neither of which care much about grooves (they will fill them where they happen to meet the glueline.)

Typically make some sort of witness marks as you where you want the top, remove it, glue or caulk on the top of the vanity, set the top in place following the witness marks, let it cool or cure depending which type of adhesive was used.