I am sure you know this - end-to-end encrypted messaging app like WhatsApp is that the messages are unreadable to anyone who intercepts them. Are you aware of this (read below)

 If you keep those messages on your device, or worse, you back them up unencrypted to the cloud—be it iCloud or Google Drive—you open the door for authorities to obtain them with court orders, effectively making the original encryption pointless.


Check this incident:
https://motherboard.vice.com/en_us/article/zm8q43/paul-manafort-icloud-whatsapp-bad-opsec-witness-tampering