I also believe that shalom bayis is more important than tznius. But here’s an interesting thought:
What would you do if your wife put on a burka? Would you just accept it for the sake of shalom bayis? Or would you risk a little shalom bayis to preserve your own sanity?
If you pick the second option, can you also understand that to some people, wives dressing tzniusdik is an issue of sanity for themselves, too?
And I’m a borderline atheist! And I still say this because I believe in being honest to one’s self about these issues, and only this way can one discover the truth (if it exists).
Full disclosure: If my wife put on a burka, I’d bite my tongue (as difficult as it is), just as I would want her to bite her tongue when I don’t dress the way she expects of a ben torah (as difficult as it is).