Have a look at bb_spi_lcd (BitBank SPI Color LCD/OLED library) by Larry Bank on github ( https://github.com/bitbank2/bb_spi_lcd). I used it to connect a 2" ILI9225 Color display with the XIAO using SPI and it appears to work quite well. The library supports the ST779 although I have no experience with that display.
On Sept 22, another question was posted about the ST7789: # XIAO Library Issue for TFT_eSIP with Arduino. Worth looking at I should think.
Good luck