I'd consider booting the tablet, logging on, installing a free space wipe utility (there are several on the Play Store), letting that do its job, then doing a factory reset/erase again (to erase your account data.)
That way, any data that is left behind would be overwritten.
On newer tablets, if you have root, you can use a utility called LagFix which issues a TRIM command, completely and utterly erasing any data that was deleted with no chance of recovery. It does the same thing as a free space wipe, but a lot quicker.