That recipe actually says there are two ways you can prepare this:
There are two ways to do this: throw just about everything in a pot of
water and simmer, then add the milk, tea and sweetener and brew for a
few minutes; or, let the spices soak overnight in the water then
simmer in the morning with your tea leaves and milk.
(emphasis mine)
Have you tried the latter? That way you still only have to heat things up once.
I'd be hesitant to boil milk with spices and then store it cold, reheating again. If you're using dairy milk, that is. If you're using anything other than non-fat (skim) milk, you'll get a "skin" on the top of the milk when you're boiling it - that's one of the classic signs of masala chai for me. But anyway, it becomes substantially less appetizing if it has time to cool into globules and is then reheated again. I'm not sure what problems, if any, you'd encounter using non-dairy milk like almond or soy.
You could dump the pre-ground spices into your brewed tea with milk and sugar, but the flavor profile will be a little different. Boiling the spices with the milk and tea gives everything time to meld nicely and provide a really smooth flavor blend. But I don't see any especially compelling reason that you couldn't do it.
I would like to point out, though, that boiling enough water for a travel mug's worth of tea doesn't take any more time to do on the stove than it does in my electric kettle. Adding milk shouldn't make too much difference; I'm willing to bet that after you get familiar with the routine, you can follow this recipe and still have it take 5-6 minutes. :)
Purists will insist that the spices must be boiled with the water, but those are the same people who would scoff at you adding cold milk to your tea in the first place. There's no reason why you can't, other than it might cool down the rest of your tea.
Best Answer
The only way to significantly reduce caffeine content is to use decaffeinated tea or "herbal teas." I know it's probably not the ideal solution, given the limitations on variety and flavor.
There are various claims about which types of tea have more or less caffeine, but the reality is that there's a huge amount of variation in individual tea varieties and blends. Caffeine content has to do with how and where the tea leaves are grown, more than the way the tea is processed afterward (and it's the processing that determines whether a tea is black or green or white or whatever).
For example, this study notes, based on 20 different commercial teas:
Each sample was brewed for 1, 3, and 5 minute times, so most of the lower numbers resulted from the 1 minute infusions, while the high numbers came from 5 minutes.
Granted, this is a small sample, but you can find other similar studies that come to the same conclusions. (This study measured caffeine in 4 different ways and found the white tea sample to have the highest caffeine of all.)
I believe the idea that green and white teas have less caffeine comes from the fact that such teas are often brewed at lower water temperatures (steaming rather than a full boil), which will in fact reduce caffeine content somewhat. But if you brew all the teas at the same temperatures, you'll find little consistent differences among tea types.
Caffeine concentration does become greater with longer brewing times, but if you brew the tea for a shorter time, you're likely need to use more tea leaves to create sufficient flavor, which can negate any reduced caffeine effect from the shorter brew.
A similar thing can happen with temperature. Yes, caffeine will dissolve more slowly by brewing at a colder temperature (see here for a detailed experiment showing the rates), but so will many other flavor components. You'll obviously need to steep cold-brewed tea longer, and that longer steep may counteract the fact that caffeine dissolves more slowly. Whether you can end up with a cold-brewed tea that tastes like what you want with a lower caffeine content will likely depend on your exact process and your exact tea leaves.
In any case, if you're actually trying to reduce your caffeine consumption significantly, I'd have to recommend decaf or herbal teas. Some tea companies may publish statistics on how much caffeine their "regular" tea contains, but it's hard to say how reliable these are, how consistent they could be from year-to-year, or how they would be impacted by different brewing methods. And short of having that sort of information on brands, the only way to know whether switching your tea/method has an impact would be a lab test.