using esp8266 feather to burn bootloader on redboard
The RedBoard has an FT232 chip for the USB interface.  Flashing the bootloader on the main MCU will have no effect at all on your problem.

While, yes, it is technically possible to use the ESP8266, it's not easy and, since it won't cure your problem of a greyed out port option, isn't worth the effort to try and do it.

Instead, you should be looking at things like your USB ports, USB cables, FT232 drivers, etc on your computer.