@zapta - Libraries - ST7735_t3 library is installed with Teensyduino. Not sure if we have released a new version of Teensduino since Paul took in my previous PR, which added support for running the ones without CS pins. I know there is still one outstanding PR for T4 (
https://github.com/PaulStoffregen/ST7735_t3/pull/3)
Most of the still pending PR has to do with T4 when you do something like: tft = new ST7789_t3(...) where the object is created in the heap and issues associated with DMA and the like from this memory...
To the questions
1) I believe that there may soon be a new Teensyduino beta release of stuff that Paul was working on to better work with Serial monitor and the like. Not sure exactly when yet... But it will have both libraries in it...
2) The library has the functions setRotation so yes you can get all 4 orientations.
3) The ST7735_t3 library does have the concept of frame buffer, like I have in the ili9341_t3n library. You can turn it on and then all graphic primitives go to this memory instead of the screen. Then you can call tft.updateScreen() to update it from the frame buffer. There is an Asynchronous version tft.updateScreenAsync() which will do a DMA operation to update screen once and a way to turn it on for continuous, but that gets you back to flicker possibility.
Note: In many cases you can avoid flicker without using frame buffer, by carefully planning out your graphics, such that each pixel that is updating is only updated once...
4) Assuming you get your AToD working properly, the updating the display should not impact it... Of course if your AtoD code relies on your main loop to do it and your main loop is waiting for display to complete, that may be something to look at, but the update code should not be doing any disabling of interrupts.
5) Arduino IDE - It gets the job done. There are others who prefer to go to other IDEs to do things. Sometimes I do it all within sublimetext, othes use VisualStudio...