Yeah, it's like a double-edged sword when they start asserting themselves. Gotta be consistent and firm but also give them some wiggle room or they'll get overwhelmed.
I find that giving simple choices and lots of explanations help too. Like you said, they're like sponges so might as well make the most of their absorbency!
Tantrums - yikes. I just let them be and keep my cool. They usually realize quite fast that no one's gonna give in so might as well chill.
Their curiosity is SO adorable though, especially when it comes to new skills they suddenly pick up. Encourage and provide resources, the learning bugs will eventually bite!