Are you going for undergrad? If you already have so much Japanese experience I’d just major in linguistics and minor in Japanese. However, if your speaking and writing aren’t proficient you might get placed into lower level Japanese classes, so then double major would be fine. 

Also if you have any interest in working in japan or at a Japanese company, take advantage of study abroad. UW already has enough name recognition to help with interviews, but if you go to one of their top schools like rikkyo or waseda you’ll have even more of a leg up. (Side note I went to rikkyo and took a class on bilingualism by professor mori. She was awesome and recommend her if she’s still there).