[OnePlus 3T Questions & Answers] Zegnalabel:
Many thanks Joseph for your sound advice.

After taking some time to read through all the links and sub-links getting to know the varied means of backups available, I have decided that my backup strategy will be as follows since non of the methods offer comprehensive backup and a mixed of the lots will be required to get as fool-proof as possible, before embarking on a data wiping journey with bootloader unlocking.

(i) I first perform a backup using google native backup and restore function under setting no matter how limited it is, to be use as a safety net.

(ii) Next, adb backup. The limitation with adb backup is that since Android 6.0, it obeys AndroidManifest.xml and will not backup data of those apps which has android:allowBackup set to false. The challenge is, I will be held in the dark which app falls into this category when performing adb backup and won't know until after I had done the actual restoration. Its like a blackbox you won't know what you gonna face until the aftermath had hit. Therefore to get around this, I'll be using Helium app (a wrapper around adb backup) instead of the actual adb backup since both methods refer to AndroidManifest.xml when deciding which app data it can or cannot backup. And Helium shows exactly which app it can backup vs that which it can't. Good info there for a start.

After installing Helium and activating it via PC client, I found out it does backup:

A) SMS and call logs

B) User dictionary

C) Accounts

D) Bunch of apps and their data

List of apps and data it can't backup:

1) Contacts

2) browser bookmarks

3) Calender

4) System settings

5) APNs

6) Homescreen shortcuts

7) Widgets

8) Alarms

9) Photos

10) Music

11) Videos

12) MMS

13) Google AUTHENTICATOR!!! - I have over 15 profiles that I'll have to re-setup one by one... what a nightmare.

14) Line

15) Whatsapp

16) wechat

17) Signal

18) Nova launcher - luckily its setting can be exported and backup via the app's functionality.

19) QQMusic - all my thousands of songs! - even if I back this up manually using ES explorer the app's internal linkage between playlist to songs will be broken. Will have to re-download all the songs to repair linkage and this means wasting another month of subscription fees for songs re-download. This is one of the most problematic app but it has DTS plagin which makes your music sound fantastic even with cheap earpiece, so I am putting up with it.

20) Skype - not much chat in there so that's fine to start over.

(iii) After backing up the limited stuff using Helium, I shall proceed to use another app to backup (1) to (12). And that app will be "MyBackup" (https://play.google.com/store/apps/details?id=com.rerware.android.MyBackup&hl=en). The app developer confirmed in its app's features that (1) to (12) are supported. Note "MyBackup" does not backup any app data at all when device is not rooted therefore step one using Helium is still necessary.

(iv) Next, to circumvent the limitation with failure to backup app & data for (13) to (20), my initial thought were to patch each app's manifest using this method (https://xdaforums.com/android/software-hacking/guide-how-to-enable-adb-backup-app-t3495117). But then, as I studied further, I realized another chicken and egg scenario. That is, the decompiling, editing, recompiling and signing of the patched apk would need to take place first before using the app and not after. So in another word this method with each newly patched app will not seemlessly replace the existing app but rather the existing app will first have to be uninstall before the patched app can be installed, and that very act would mean losing existing data altogether.