Pashuta- while I appreciate your attempt to make a distinction between chumrah/mihag and halacha, in this case I disagree with you.
1. Nodbody says women should dress ugly out of the house, rather her honor is in her house.
Can you provide a source that says that women should dress up when they leave the house?
2. There is a concept (which is halachic especially tznius) where keeping the rules only as specifically stated is not enough. There is something called ruach hatorah- not to be a minuval b’rshus hatorah. I think we all know this is very relevant to tznius. as there is even the concept of daas yehudis, things which are not written in the shulchan aruch but must be kept. An other example could be students copying from one another, it is not clear halchicly if it is technically genvas daas. (it might be in the case of college students but hard to make it for younger students as they are not getting an monetary compensation for their papers/tests this is a subject I have done qutie a bit of research on ) however, i think everyone would agree that this is not behaviour a jew should do. there are many other examples, where the Torah does not forbid something but it’s obvious that one is not suppose to do it. A mashal. If a husband and wife have a strong good relationship, the husband will know not to do (and to do) certain things with out his wife having to tell him and spell out specfically. Hashem told us eloyki elo sonah zima- Hashem loves kedusha and tznius kedoshim tehyu ki kadosh ani.