What it looks like and how it will act aren't really tied together
I don't know much about shih tzus/bichons but I have a corgi mix and she is pretty high energy but she seems to be mixed with another type of potentially high energy dog. She calmed down a lot when we got a second dog though and had someone to play with a lot. Anyway since dogs temperament does vary even within a single breed, I am not sure what you can predict of a multi mixed breed.