Chosen Solution
before all of this happened, my iphone was freezing every 5 minutes or so, and would only unfreeze if i plugged it in for a few seconds and then it would go to the lock screen. Then, i was stupid and updated to 12.5.6, and it was stuck with the progress bar at 99%. Then, it rebooted, but when it reached 75% it would reboot, and every reboot it would reboot faster, until we reached this stage. I dont have the crash logs anymore, but the error code was WDT and it was something about no battery data on the end (?).
Based on your initial panic, my inkling is that it could need a new battery. But the Apple logo shows and then goes super dim also points to a possible issue with the Ambient Light sensor. I would give a battery replacement, and a replacement of the front facing camera assembly a go to start and see if that sorts your issue. The iPhone 6 plus is one of the easiest to repair iPhones. No adhesive between you and the guts of the phone which is a win on the repairability front anyways.
Another user suggested battery and display replacement, but I have a bunch you just need a good restore. Perhaps it’d be something you could try before replacing parts? Connect your phone to your computer. To place your device in DFU mode, begin by holding the power and home button until the screen goes blank. Release the power button and continue holding the home button. Your computer (itunes more specifically) should recognize a device in recovery mode, allow it to restore your phone as normal.
