Settings, App and Messages Backup AND restore of non-rooted Android 6+ devices?
Question and answer you refer to (Full Backup of non-rooted devices) are mine – so I can assure you they are still valid. As for the "built-in" backup: that's usually backing up to Google's servers, which works pretty well. What has mixed results, however, is the restore of that – which the user has very little means of controlling. It either works on first startup and configuring the Google account, or it works partially, or not at all.

I'd suggest you follow the procedure described there, and backup your contacts and messages separately. You can find some suitable apps in my app lists.

If your device were rooted, you could simply use Titanium Backup and be done with that, but I understand that's not always an option. If you have a backup device, I'd additionally suggest doing a "test restore" to that, to see that your crucial data can be restored properly. WhatsApp is a completely different case; though it has its own built-in backup, restore to a different device is tricky at best. If you don't mind (and using WhatsApp suggests as much), use it's Google Drive capabilities which should make things easier. Not that I have much experience with that (I wouldn't use WA unless threatened really hard).