How would I address my niece in Japanese?

I’d say it’s vibes-based/comfort-level with the addressee. Which can only be understood thru immersion I believe.

I liked to joke with my coworkers and call them name-dono/-sama every now and then. I earned the coveted oniichan from only 1 of my coworkers.

Yes, exactly.

It took me a while to understand that the calculations are based on social calculus and not arithmetic, but you do get it after a while.