Vietnamese Coffee (cocktail) - what to sub for condensed milk?
I would suggest that you use cream of coconut, which is the base for piña coladas as a substitute. It's thick and sweet, and should complement the avocado pretty well. Note that this is a canned produce, the most widely available brand in the USA is Coco Lopez.
Outside the USA it's harder to find, and can be pretty expensive. There are also products that are confusing, like creamed coconut, which isn't remotely the same thing. I make my own by spooning the cream off the top of coconut milk and mixing it with some palm sugar (regular sugar works fine too) and then cooking it down to thicken it.