You can back it up automatically to your e/murena cloud using the Nextcloud app.
First, go to https://murena.io/apps/files/ and create a folder called “Whatsapp backup” (or whatever you like).
Then, on your phone, install the Nextcloud app and open it. Choose “Log in” and use “murena.io” as the server address. Log in with your murena credentials. Make sure you set the permissions the app ask you to.
Finally, press the menu button (upper left corner) and go to “Settings” → “Auto upload”. Press the 3-dots in the upper right corner and choose “Set up a custom folder”. For the “Local folder” use “…/Android/media/com.whatsapp”. For the “Remote folder” use the folder you created at the beginning. Set “What to do if the file already exists?” to “Overwrite remote version” and press “Save”.
Now, if you reset your phone, first install Nextcloud and set up the backup like above. Then, install Whatsapp and your conversations will show up automatically.