The short and simple answer is you should uninstall or delete SdFat you've installed, so Arduino IDE will use the copy of SdFat which comes with Teensyduino. When you install a library, usually Arduino IDE gives your installed copy priority...
It might be not the code, but power. If you have low quality USB cable, it has significant power draw. When you are using extra devices, like display etc, the teensy gets even less current.
Normally I don't comment on future products, but I can in this case confirm no new Teensy is currently in development. The focus for 2024 is software, both new features and catching up to a massive backlog of contributions and suggestions and...
PJRC is not currently in a financial position to launch any new Teensy using an expensive chip like RT1176.
Teensy 4.0 and Teensy 4.1 have been continuously in stock since November 2022, and we had them in stock about 75% of the days in...