We can remove data permanently from android flash memory, which is encrypted, after a factory reset.
After doing this, do we need to decrypt the device for using it again or factory reset the device?
It is in the default factory reset state ( like a new device ). You can't decrypt the device even if you wish to as the encryption password (generated from a combination of user lock screen PIN and encryption algorithm ) is lost
Encrypting does not completely delete the files, but the factory reset process gets rid of the encryption key. As a result, the device has no way it can decrypt the files and, therefore, makes data recovery extremely difficult.
When the device is encrypted, the decryption key is only known by the current OS.
If you know the pin (for that screen you get before the animation starts), the device can be decrypted and get booted up successfully.
If you don't remember that, then you will need to reset the device. This resetting is similar to a factory reset.
Factory reset will not retain your personal data and apps in the internal storage.
Your external (micro) SD card, if present, can be removed before doing the reset only if it is configured as a portable storage in Android.