I know there are screen covers to block visibility for any angle except straight on (like for bank tellers) but is there a mode to do this? I see Dark Screen is an option but it is grey and not available to swipe on.
No. What you are describing is reducing the viewing angle, which requires the screen to do so physically. AMOLEDs have great viewing angles. Dark Screen does not reduce viewing angles, but prevents people from spying on your phone when you are away by not letting it light up (enable-disable by pressing twice the power button)
If you search on Amazon (or wherever you prefer) You'll find Privacy Screens https://www.amazon.com/protector-HYAIZLZ-Hardness-Tempered-Protector/dp/B06ZYLLWSB
It would be super cool if Samsung implemented a "privacy" mode, but unfortunately not yet. But as mentioned above you can purchase privacy screens from amazon/ebay. Its like a screen protector, as well as a privacy screen so from certain angles (usually side) people cannot see what you are seeing.
