Thank you so much Duragrouch !! It's been a loong time, and I really liked your explanation that I'd need a different derailleur or a hangar for a wide 1x cassette.

What I did:

1. Bought 3 different sizes of derailleur extender. ( they were not giving the results I wanted as it was causing the derailleur to hit the tyre). Tried adjusting the b scree, shorter chain. Nothing worked.

2. Was fed up, then just decided to buy the exact derailleur that Ron had. (New Shimano Deore XT M6000 for around 40 USD).

It just worked. No faffing, no playing around. It just worked.

Like how a snake will bite its victim until its venom and victim are finished; I did the same, by troubleshooting and measuring with advice from you and others on this forum.