The teensy is so weird though. At least back when I played with them. They put a secondary chip which let's you flash over USB but they cover the debug pins and the only way to get serial over the USB port is to have a whole USB stack as part of your application. As a development board I would rather go with one of those STM32 backed boards and a knock off STLink, you need the STLink to flash, if you want DFU you can add that yourself, and you get a debug adapter.
The teensy is so weird though. At least back when I played with them. They put a secondary chip which let's you flash over USB but they cover the debug pins and the only way to get serial over the USB port is to have a whole USB stack as part of your application. As a development board I would rather go with one of those STM32 backed boards and a knock off STLink, you need the STLink to flash, if you want DFU you can add that yourself, and you get a debug adapter.