I'm equally thrilled that you will not only write this pairing, but that you seem to have an understanding of the dynamic in a way that I find deeply satisfying. (Not that there's any one way to view the characters mind you or that my own take is in any way "better" it's just a matter of preference.)

I cannot feel that these two always have such a serious evening, and that they do play a lot, but sometimes Sirius would need this kind of catharsis.

Same, and in my own take on this sort of thing, I view Sirius as giving Severus "permission" to find that playfulness and indulge it, where Severus gives Sirius room and incentive to find those parts of himself that are more serious and focused. Again, it's just rewarding to bang those two ways of being off each other.