Stepparents who entered the lives of their stepkids early: How the dynamic has changed over time?

I became a stepmother when my stepson was 3,5. Now he is 4,5. Because he is young, he is really demanding and sometimes emotionally draining. Obviously, when he is with us (every other week), he requires almost 100% of his dad's attention and wants to play with him all the time. Apart from the challenges linked to living with a toddler, he has a really, really difficult HCBM, which we can’t avoid much because of his age. Right now, it feels like it will be forever, but I know it’s temporary because the child will grow older. However, I wonder how it will be.

What is your experience? Which age was the most challenging or the easiest? How have things gone for you over the years?