Depending on your threat model, your check is insufficient, since dd |hexdump will be all zeros even if you just trim all the blocks for a drive that is trim-to-zero.

Securely erasing flash drives with a threat model of "someone will dump the raw data of the chips" is only fully solvable for self-encrypting drives where you can replace the key. Even if you can issue a block-erase for every single block of the device, block erase doesn't always succeed on NAND.