I don't see how it is necessary to delete any accounts (Step 1) using the steps you outlined in your comments. However, if you're using Android 5.1.1+, you likely want to avoid Factory Reset Protection. In that case, it's just necessary to delete the main Google account tied to the device. (I added parenthesis around two of your steps that I think are unnecessary):
- (Delete accounts) Just delete the main Google account tied to the device if your device is on Android 5.1.1+
- (Encrypt) This is the default in Android 6+
- Factory reset
- Install and use data shredder
- (then encrypt and factory reset again)
The account tokens are encrypted in Step 2 and are therefore garbled for any bad actor trying to recover your information.
When you overwrite the free space in Step 4, the original master encryption key may be overwritten (and any space that is not 'free space' was already overwritten by the factory reset). Android may store the key in an area that isn't reached by an eraser app that overwrites free space; however, the free space that originally held the encrypted data is overwritten by the eraser and thus even a bad actor who has access to the master key won't be able to decrypt anything. Therefore, any subsequent encrypt/resets beyond the original Steps 2 and 3 are pointless.
Updated the answer with Robert's comment below... and for a second time.