We are writing to let you know that you will be out of Google Account storage once your WhatsApp backup begins counting toward your 17 GB storage limit in at least 30 days. Learn about WhatsApp backups on Android.

The solution suggested by Google is disabling backups with Google. To move to another phone they have this tutorial.

But... Well: it's probably better to keep some kind of backup. My best idea so far is creating a new Google account just for the WhatsApp backup. Is it possible to sync that WhatsApp backup with another account? If so, how? Any other better solution?