“It is a day of intense Holiness. not a day of taking it easy and recuperating from the week.”
Respectfully disagree, and suggest it is both. We celebrate the cessation of the work of Hashem by emulating Him and not working. C”V Hashem needs to “rest” or “recuperate” (as you point out), but the concept was created for us people who need a “break”.
How to spend that break is a seperate issue. The Gaon was known to learn extra, as “V’hashaina Meshubachas”, he needed less sleep. What better way to celebrate Shabbos by Giving Daf & learning extra, spending quality time with your children, and realizing what a bracha it is that Hashem created the concept of rest from the “Sheshes Yamim T’avod”.
As far as the clothes issue, I have not weighed in because I don’t know the circumstances. I would consider a polo (or non-white) shirt unacceptable for myself, but I have neighbors who do wear them on shabbos & have no issue with them either. Others would only wear a bekesha, and look down at those who wear a suit. Besides, I’m not sure if the issue is Shabbos or a Shiur.