This looks like another one of the displays without a chip select pin?
Is the issue that you are simply wanting to mirror or that your display is showing stuff mirrored?
Anyway most of that stuff is controlled by the method setRotation(m) where m is 0-3 to rotate the screen in 4 orientations.
for each orientation it sets up the displays register MADCTL which controls how things are stored in memory and when it increments how it increments...
So you can take a look at void ST7789_t3::setRotation(uint8_t m)
And see how it is currently set and experiment some.
You can either change these sources and/or create your own subclass of it.
That is the ST7789 code is 99% the same as the ST7735, mainly only some defines different and a couple functions, including the rotation function.
So you could simply subclass this class and overwrite the setRotation method.