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.


Increase the amount of tea. More leaves, means more stuff extracted.
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.
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.