There is no need to feel these need to be balanced because you can be nice, kind and assertive at the same time. Some people think when they are being assertive they are not being “nice.” They may equate being nice with being a pleaser.
That means doing what others want or expect you to do. Then you are not honoring yourself or living your truth.
I believe the kindest thing is to be true to ourselves and not take on the response/reaction of the other. Years ago when I was taking ballroom dancing a gentleman asked if I would like go for coffee. I gave him a warm smile and kindly said, “No thank you.”