Some Initial Questions:
1. Are leather dress shoes needed, or can I get away with nicer sneakers?
2. Button downs, how bold or subdued should the pattern be?
3. On button downs, top up done or not done? I've seen it go both ways.
4. Button downs designed to be worn untucked - yea or nay?
5. Belts? I know the ones with a more mettalic / matte look are becoming popular over normal old leather and buckle belts, but I'm never sure if they fit business casual.
I used to work at a Midwestern investment shop with "business casual" dress code (most of the time, anyway). For your questions, based on our pre-pandemic workplace:
1. You cannot get away with sneakers, even nicer ones. You don't necessarily need Oxfords or brogues every day, something
like this would work (including less expensive brands, they're just my go-to shoe reference). In retrospect I should have had some less-formal-than-dress-shoe-but-more-formal-than-sneaker options.
2. This will depend culturally. I would describe the traders' choices as variants of white, blue, gray, pastel on occasion, and patterns such as stripes and checkers. Earth tones likely acceptable as well. The father you got from that area, the more leeway there was. I'd lean towards you can dress sharply, but shouldn't make yourself the center of attention just because of your dress.
3. Top button should be undone unless wearing a tie, which you wouldn't be unless something more formal was going on that day. Too stuffy to have the top button buttoned without a tie.
4. At least where I worked, no one wore button downs untucked, not even on casual Fridays when the non-trading-floor areas allowed polos. Maybe if you meet some colleagues after work for drinks?
5. Matte texture on the belt itself? If I'm thinking of the right thing, I have one of those that I've worn at future (post-investments) employers, with a more relaxed dress code, and it can look spiffy. Didn't have it when I worked there, but I say people on the IT side wearing them, but not on the business investments side. Probably depends on the shop.
In general though, I'd say lean towards the more formal side the first couple weeks, and if you find you're overdressing a bit, it's always easier to loosen up a bit than to start too casual and have to erase that initial impression.
But I'm also suspecting your future employer is a bit more relaxed than my investments company employer was, given that the CEO was in khakis and a polo. I can't imagine our Chief Investments Officer wearing a polo to work. What I've described above is really 2017 conservative-employer business casual, and standards are likely more relaxed post-pandemic at most places.
For fun, my (IT) team at that place of employment would also dress like we were in
Mad Men once a month, three-piece suites, or two-piece with tie bars, traditional white shirts, with cufflinks if we had them, formal dresses for the ladies. But that was very much business formal! Mainly we just wanted an excuse to dress fancy, and on those days we did indeed out-dress most of the investment professionals, the exceptions being those who had very important external meetings.
Business formal meets casual.
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Is that Yevgeny Prizoghin in casual? With a head of state from the African delegation to Ukraine and Russia in formal? With the hotel maid wearing the stereotypical attire for a fancy-ish Russian hotel maid?
Prigozhin's attire would be acceptable in the non-trading-floor parts of the company I worked for on a casual Friday, although an undershirt with the polo may be appreciated. The head of state's attire would be appropriate if a high-level government official, or high-level external investment professional, were visiting the office on that day, and overkill otherwise. Indeed, business formal and not business casual.