I believe it was around 2011 that the more premium bubble tea vendors in Singapore started offering a choice of how much syrup to add to one's drink (Koi Cafe, which may or may not have originated from Taiwan, might have started this trend).
I'm surprised that this particular service hasn't made its way to Japan.
The brown-sugar tea is all over the place and if you order some green-tea with 0% sugar they look at you like you're an alien and tell you it will be too sour and not taste good.
I lived there for 5 years and my word it was tough to cook for yourself. Lucky I lived near a wet market, but otherwise your choices aren't greay
I actually try to remember the eating houses that offer ice lemon tea without sugar (very rare). Usually it's even better quality/fresh at that.
The big thing lately in Singapore, Tokyo, Malaysia is "brown sugar milk tea". I think it's hit NYC and LA but basically you take a tapoica milk tea but before you put the milk and tea in you coat the sides of the plastic cup with brown sugar syrup. Then, instead of the old tea + milk it's now like tea + cream and far more cream than tea such that it's more like sweetened tea flavored milk than milk flavored tea.
Search for "Tiger Sugar" for I guess the brand the started the new style?
https://www.google.com/search?safe=active&q=tiger+sugar&tbm=...
There are apparently the equivilant of 20 teaspoons of sugar in 1 drink. Compare to 330ml of Coke which is about 7 teaspoons of sugar. The Malaysian government put out a warning as they got super popular.
https://www.malaymail.com/news/malaysia/2019/07/25/health-mi...
I'm curious why now in Tokyo. Tapioca Milk Tea was popular in Taiwan in the 90s and made it Los Angeles in the late 90s early 2000s. People tried for years to bring it to Japan and I was told it was too sweet for Japanese tastes. But, now it's even sweeter and has some how exploded in popularity to a ridiculous level. It's like every spare store front has started selling.