About what bounderies need to be set, I think you should be more lenient with those that are officially OTD/non-religous. It is wrong to expect a girl dressing normally in pants to show up to the shabbos table wearing a skirt that is 4 inches below her knee. Tell her to wear clothing that is looser, short sleeves, and either pants that aren’t tight or a maxi skirt. (IMHO, I think those are much more tzniyus than a skirt above the knees, when it comes down to not being michshol a man, which is the important thing here)
Also, when giving rules you must clearly state WHO the bounderies are for (hint: they are not for your 21 year old OTD daughter who really doesn’t care about spiritual rules) and WHY, the reasoning behind them.
Instead of saying “You must come to the shabbos table respectable, cuz you need do be more tzniyus for shabbos” say “In our home we dress to respect those around us, and you need to wear respectable clothing (define what that means) outside your room at all times. Shabbos is special to this family, so if you would like to wear nicer clothing to the shabbos table either to respect shabbos or the family, it would be appreciated”
Instead of saying “We don’t play goyish music loudly cuz it’s assur” say “We do not allow non jewish music to be played loudly be anyone in this house because me and spouse feel that it has a negative affect on the avirah”