Severe powering troubles due to Micro USB power connector. It’s recommended to power through GPIO pins to prevent under-voltage issues (instabilities, boot/crash cycles). Powering situation is a little improved/masked on model S.
serial console is enabled on UART2 (pin 32, 33, 34=gnd)
eMMC flashing (model Tinkerboard S with eMMC) can be done the same way as burning an SD card. Connect your board with USB cable to your computer (Linux or Windows) and eMMC will be seen as a new hard drive. Burn an image to this drive. Wait until it's done, detach from computer and power with your normal PSU.
booting from SD card on S model is possible in Maskrom mode.
