make something that zeros all the files and deletes them, deletes and overwrites all the contacts

No, there's no need for anything that complicated.

In principle, encrypting all data on a phone is really simple to implement. In practice, it's carefully thought out to avoid edge cases. The general idea is something like this:

   when first activated, the
   phone generates a random 256-bit AES key

   phone uses this random AES key to encrypt
   all data stored on the phone

   phone retains this random key in a special
   location, and encrypts this key by using
   the user-provided PIN

To quickly erase all data on the phone, all that's necessary is to overwrite the key in the special location with random data. From that point on, there is no feasible way to recover anything on the phone. Period.

It isn't necessary to erase an entire device. It's merely necessary to replace a 32-byte field (that contains the true AES key) with 32-bytes of random data.

From then on, it doesn't matter what the PIN is. Data on the phone is jibberish unless and until the proper 256-bit AES key is produced. That key no long exists, so from that point on the only way to recover the data is by brute forcing AES, by trying all possible 2^256 combinations.

They look like you had a bunch of things and then erased them

No, what remains is indistinguishable from the case where the correct PIN hasn't been provided. Having "things" on the phone is no evidence of guilt. There is absolutely no evidence that the phone was erased. All that is known is that the provided PIN isn't able to decrypt the data.