Try "fastboot reboot"

EDIT: That won't fix your bootloop though. "fastboot reboot recovery" might work to get you to recovery, where you could possibly wipe the TV. No idea if the Bravias even have access to recovery, although they should.

This seems to suggest a procedure exists for reloading from USB - https://community.sony.co.uk/t5/android-tv/sony-bravia-android-2015-firmware-rollback-procedure/td-p/2091088

Not sure if that's valid for your specific model, nor where to download the USB image for your model. Some Google-foo should get you that info though.