When I make bubble tea at home, I just brew up a big ol' pot of tea, add some vanilla extract and a light dusting of cinnamon (since the mister used my last cinnamon stick to make chutney and I haven't been bothered to buy more), and add just enough milk to change the colour. I put my pearls in straight from the pot without bothering to cool them down, add ice, add tea, and it all tastes fine.
Just a word of warning: pearls do NOT keep in the fridge. The consistency goes terrible, and pretty much nothing you do will redeem it. With that in mind, do not make more than you need, and if you do accidentally make too many, try to eat them all (but be warned that they are surprisingly filling).
As to avocado tea, my folks used to make a dessert out of mashed avocado, milk, and sugar. I imagine you could use this mush to flavour your tea before adding bubbles, but as I'm allergic to avocado, I haven't tested it out.
Good Asian groceries will usually have bulk packs of tapioca pearls as well as the instant stuff (and in different sizes and colours too), jars of jelly, and the motherlode: flavouring essences. I've yet to try it out (I've been banned from buying more baking gear until I've gotten rid of all the stuff we already have), but they have lots of different flavours. I'd like to try a pandan bubble tea with grass jelly.