To remove the Activation Lock from your iPad, follow the section entitled "If you no longer have your iPhone or iPad":
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
As for the security of your data, iOS/iPadOS is architecturally designed to protect the data from being accessed by anyone other than the owner.
All locally stored data is encrypted; by design, the only copy of the encryption keys necessary to access local data are held within the Secure Enclave - the device security chip. The device Passcode unlocks the Secure Enclave, which in turn releases the encryption keys to the Operating System while the device remains unlocked.
If an incorrect Passcode is repeatedly entered, the Secure Enclave automatically wiped and the device disabled - an operation that erases all stored encryption keys. This is known as a crypto-erase. Once erased, all locally stored data is permanently beyond reach and cannot be recovered.