This is so pretty! I love this name so much! I love the spelling, pronunciation, and admittedly the meaning. The meaning could lead to a person thinking that a Brynja is storng willed and not someone to mess with but the pronunciation could ease their minds because the name sounds peaceful & graceful.
We have both a Sonja and a Brynja. Most Scandinavians know how to pronounce both but it's not such a "nightmare" in school. You just tell people how to pronounce it and as you get older you can be glad for something unique and pretty.
This name actually isn't that bad in terms of pronunciation and such (in the English-speaking world). Most people, even little kids, know that Sonja is SOHN-ya, so anyone named Brynja shouldn't have too much difficulty. And I like this name, actually! It sounds quite pretty.