So I do not want to discourage you but we had a period of several RCA Doorbells brick when Nelly's firmware was applied, earlier this year. Not sure if any recovered, but that was before it was discovered one can recover from a brick by doing what you did. Have you concerned trying another firmware, I would just stay away from Nelly's. Try holding the reset button down longer after applying power, before releasing.
Argus
Had the same thing happen to me.
After reading through the 101 and various posts I did the following and it recovered.
Turn off power and wait for the red LEDs to go out
disconnect power kit box and chime if connected
remove sd card and format to FAT32
copy the HIK firmware to the card as downloaded
copy the HIK firmware to the card renamed to digicap.dav (so you have 2 files on the card)
reinstall SD card
press and hold reset button
apply power
LEDs will flash once, do not release reset button
continue to hold the reset button until LEDs turn blue and doorbell announces reset successful
This is a combination of various suggestions on here so not sure if all are necessary as I was trying different things as I read them.
The above is the combination I had when it came back to life..
HTH