I read that 要するに and つまり are not always interchangeable. Still, I don't really understand the difference between the two...
The two can be interchangeable but where as 要するに means "To summarise" or "in short". つまり is usually taken to mean "in other words":
There is a similar word to つまり、すなわち which also means "in other words" but is not interchangeable because it does not convey a a conclusion:
I explain this because (I think*) in the following sentence つまり is interchangeable with either 要するに or すなわち, illustrating the nuance you are asking about.
*I should be grateful for if others would confirm correct me on this.
Not 100% sure on this, but I think