Bitterness and astringency are different problems
They are constantly run together and they have different causes and different fixes. Bitterness is a taste, and it comes largely from over-extraction driven by heat. Astringency is a tactile drying and puckering sensation, caused by polyphenols binding to proteins in saliva, and it is influenced by leaf grade and agitation as much as by heat. Advice that runs them together will fix one and leave the other. Work out which you are tasting before changing anything.
Check these, in this order
First, water temperature — this is the answer in most cases, particularly for green tea. Second, steep time. Third, leaf quantity, because too much leaf makes even a correct steep harsh. Fourth, whether the leaf was left sitting in the water while you drank. Fifth, agitation: stirring, squeezing a bag or shaking an infuser all increase extraction of the harshest fraction. Change one at a time and taste again.
Getting below boiling without equipment
Boiled water left standing in an open mug drops quickly at first, then slowly — a few minutes gets you into green-tea range in most kitchens. Pouring between two vessels drops it faster. Adding a measured splash of cold water is the most controllable of the three. None of this needs a variable-temperature kettle, and precision to the degree matters far less than being deliberately below boiling.
When bitterness is supposed to be there
Some teas are bitter by design and the bitterness is the point. Young sheng pu-erh from Bulang is aggressively bitter, and the sweetness that arrives seconds later — hui gan — is what it is drunk for. Matcha carries a bitterness that is structural rather than a fault. Dan Cong oolong sits a few seconds away from bitterness at all times and is brewed against that edge deliberately. Judging these by the standards of a mild breakfast tea misses what they are.