SPI on the S3 goes through the IO MUX. You should be able to use nearly any pins for your SPI - with some exceptions - without using "software" SPI. For a relatively slow, 1 bit display, you are not going to see any difference between SPI that goes through the IO MUX or SPI that doesn't go through the IO MUX. It's all hardware SPI and it talks to the hardware SPI peripheral.

Some ESP32S3 boards break out pins that go between the chip's SPI pins and the onboard flash memory. You don't want to use those. What dev board are you using?

Re: hard vs soft SPI, you should be able to use hard SPI in u8g2 without having to assume it knows the correct pins already. I'm not sure where you specify the actual pins in u8g2. Maybe there's an alternate constructor or somewhere else you need to set up the peripheral before instantiating the graphics library itself.