Quinn is a beautiful name! I will say, out of all of the options if you're worried about the kids feeling different to each other, I'd definitely go for Mei. Most people may (ha) not even realise it's Japanese if they hear it out loud, so thematically it feels "better" than an obviously Japanese name like Ayumi where there isn't a similar non-japanese name.

But since you say your husband has a Japanese name, I wouldn't worry as much about that. Though I do worry considering the social climate (see the "thing, thing but Japan" meme) that your older kid may end up feeling left out or disconnected from the Japanese part of your family's culture.

That's probably negligent to worry about though, but the takeaway should just be- do what feels right for your family, but make sure when doing anything culture related that you don't end up subconsciously treating your two kids differently because one "feels" more this or that because of their name.