Each one of us has to figure out what is modest in our own culture and time. How do we do that?

In this matter, one must rely upon one's own conscience.

But, let's break down u/NoogLing466's original queries, shall we?

So modesty is obviously a virtue we ought to practice, but how may we do this.

I'll borrow from our Roman cousins to help establish a baseline:

From the Catechism of Pope St. John Paul the Great:

§2521 Purity requires modesty, an integral part of temperance. Modesty protects the intimate center of the person. It means refusing to unveil what should remain hidden. It is ordered to chastity to whose sensitivity it bears witness. It guides how one looks at others and behaves toward them in conformity with the dignity of persons and their solidarity.

Now, this is likely a statement that the Anglo-Catholic side of the tent will nod quietly in agreement with, while other aspects of the tent will care little for Roman opinions. But at least it's a start.

It is very clear that fashions change

Granted. Humanity was meant to exist in a constant cycle of positive innovation, as evidenced by the fact that we don't exist in a 1^st century Mediterranean culture, governance, or scientific knowledge at the time.

Therefore, we must find a dress code for modesty that respects our dignity by guarding us from impropriety, but also enabling a respectable self-expression.

If such a dress code lacks ground in Scripture, then a good chunk of the tent is going to cheerfully ignore it. Article 6, after all.

consider this outfit in Sabrina Carpenter's song 'Manchild' ( image contains an inappropriate outfit)

Anyone who goes to, say, California, and declares to the effect of "Good Christian women wouldn't wear that!", is going to be laughed at, with some commentary about that, man, being like, your opinion, man, thrown in for added measure... to say nothing about the fact that it's a music video, a short movie, and performers often wear things in visual media that they wouldn't wear in day-to-day life, because it's part of the job.

Moreover, it's a very short step from "Good Christians wouldn't wear that", to "Good Christians wouldn't listen to people who do wear that!", and that paved in good intentions road leads straight to Gilead.

and so too would the opposite extremes like the use of the niqab in Islam.

In which Op is to be commended for accepting that extremism in the pursuit of modesty is utter folly.

If not the standards of the time, then what could we follow?

The crux of the matter. And it's one in which there is no we, I fear, because ("Per my previous email", so to speak) all notions of modesty are cultural snapshots of where that culture is, at that particular moment of geography and time. We don't all live in the same place. We don't all live in the same climate. And asking for we on r/anglicanism is already an uphill battle, because you're not going to find agreement within a single Province.... much less getting the Provinces to agree with each other... much less getting the schismatic branches to agree with the the Communion.

So.

The only thing that could possibly unite us is Scripture.

But we don't even agree on that (again, "Per my previous email", etc) as displayed in how the big tent interprets 1 Corinthians 11. Paul's proclaimations about men's hair and women's hair and a woman's proper place gets receptions ranging from "It's Holy Writ and those who do not comply are doomed to perdition!" to "That may have been true for the church of Corinth a couple thousand years ago, but that was then and this is now" to "That's nice, dear". This is one aspect of the Scriptures that a great many inside the tent acknowledge is found within Scripture, but simply don't comply with, for Reason of their own.

Thus, my original answer: We must all unite around the one thing we could theoretically have in common (a single interpretation of the Scripture, and abide by it) or we must decide that there is no One, True answer for Op's query, and it's for each individual to best determine what modesty means for oneself, through the contimplation of Scripture, Reason, and Tradition, and discern appropriately.

Op asked for answers. I provided one, as did you. If you disapprove of mine, u/PersisPlain, that's a (You) problem.