The APKMirror site is likely to be safer or at least just as than Google's Play Store. APKMirror is much, much smaller in scale than the Play Store so it can check and verify each APK it hosts (On the other hand, the Play Store receives literally tens of thousands of app submissions each day worldwide so it would be a much more involved process to apply the same level of vetting each app individually. There simply are too many posts with bad intentions to screen out all of them.)
https://www.apkmirror.com/faq/#Safety_and_Security
Regarding those db files, those are the actual database files that apps use to store your data. Use the corresponding backup app to import those db files or if there is an Import option in the Notes app and the Contacts app on your new Optus phone try that.
Thanks!
Is there another way to backup my SMS messages and contacts etc without Google One? Is there another online way to do it?