How to make a strong cup of milk tea without boiling it much?

As a general rule for tea, milk tea or otherwise... You can make it "stronger" a few ways. Increase the amount of tea, increase the steep time, increase amount of tea or raise the temp.

I would suggest trying it in that order.

1. Increase the amount of tea. More leaves, means more stuff extracted.
2. Increase the steep time. This should work, as essentially you're allowing more time for the water/milk to extract the flavours from the leaves. However the potential downside is over-steeping, which could result in more of the bitter tannins tastes coming out of the leaves. Since Milk Tea has a large quantity of milk and usually is a stronger tea to balance, this might be what you're looking for.
3. Raise the temperature. This might result in more "flavours" coming out, but generally, they're not the flavours you want. Most of the pleasant flavours of tea dissolve at a lower temperature than the tannins. Again, as milk tea is generally a stronger drink, that may be what you want. But I'd try this last, as you could also lose the other flavours in the tea.

Also, not entirely clear from your question, but it appears that you're adding the tea to the cold liquid? I'd probably heat the milk/water to about 80-90C before adding the tea. If for no other reason, than making it more predictable/repeatable as to how long to steep for.

I found the solution which suits me:

I first boil the milk, water, and jaggery together in quantity mentioned in the question. After the mixture reaches boiling point, I switch off the gas stove and throw in 2 table spoons of tea. I close the lid of the vessel and let the it sit on the gas itself for 6 minutes.

After 6 minutes, I strain the tea and the result is a non woody strong hot tea.

I think it is important to let the covered vessel sit on the gas because otherwise the tea will cool off in 6 minutes.

You could make a reduced syrup then use that as a base for milk tea drinks: place a large amount of tea in a pot on the stove, add water till covered and heat gently until it has reduced down to a strong syrup. Store this syrup in the fridge. When you make tea, heat the milk to the temp you wish to drink at and pour in the syrup, sugar to taste, enjoy.

you can make tea at very low temperatures but it will take more time there is something i learned froma homeless man in california, he put a tea bag, or other herbal leaves, into a glass bottle of water which he carried in his backpack, throughout the day the teas flavour was imparted into the water which became warm though the day. in the evening he had a strong tea which was luke warm, to drink.

so you could try steeping the tea overnight in just the water and then warming it over a low heat for as long as it takes to get to the strength you like. then add the milk at the end. warm milk obviously wouldnt be advisable becuase it may sour, so you would have to leave out the milk.

also i notice when making chai tea that if i put the tea and water in before the milk the tea seems stronger, i think the milk impedes the tea tannins from coming out, so you could try leaving the milk till the end. hope this helps.