As I did more testings on my Fire TV Stick 4K within TWRP, most of the issues usually occur while backing up the system and vendor. In the past, I thought I was doomed because of a bricked Fire TV Stick 4K for backing those two partitions. Yesterday, I finally discovered the true fix with backing up and restoring. You need to only backup the system image, vendor image, data, and boot to get your Fire TV Stick 4K working again WITHOUT flashing a prerooted or stock image. Let me know if this works for you and I'll be glad to respond back. I also have my version of a debloated rom for the Fire TV Stick 4K which may be available in the near future and is safely able to factory reset successfully along with the setup wizard going through without issues.