I am an absolute novice (a good way to start): I purchased a Tree of a
Thousand Stars on Friday, and keep it in my bedroom.
I have a north facing room, and so don't get much in the way of direct sunlight


Terrible. Find a south facing window or put it under a fluorescent light.

the temperature is around 18 to 22 degrees Celsius throughout the year (day
and night).

Much too hot. Should be around 15, especially at night, and outdoors all
summer.

Some of the leaves of my tree are starting to yellow;

Telling you about the above.

(I've been watering my tree pretty generously),

Stick your finger or a wooden skewer down in the soil. Don't water until it
starts to get dry.

I'd rather not remove it from my room

Give it to someone else before it dies & get an aspidistra for your room.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)