Awesome. Thank you Zaxxan. Tested as follows:
Built APK and tested on real devices.
Decompiled APK to check AndroidManifest.xml and the billing library is identified as 6.0.1.
Built AAB package and uploaded to Google Play Console.
Passed all automated checks and tests successfully.
It is now waiting for Google to review the app update and make it "live", but it has so far passed all tests with flying colours, including the checks for any policy problems (which the Google Billing Library version was listed as). I am confident it will go through fine but will let you know as soon as I hear back from Google.