@DrunkenMonk dude, I think that you're not very aware of the issues that a lot of people are having with backlight screens. It's recommended to use Stock systems, in order to avoid issues. And obviously it's more than reocommended, while updating a firmware, it can lead to more issues if the power delivery is not the one that the system and the EZFJr needs while they're rewritting important files-codes. It's very important when someone ask for help in troubleshooting to mention all the circumstances in order to find the issue and fix it, take it like " lie to the doctor ", trust me, very bad idea.
There are people with issues almost identical to the ones that you describe, but with stock FW4 & Kernel 1.04e, just because a mod (screen is the most common) in their systems.
With that said,
and please, don't misunderstand me. The last firmwares do have a LOT of issues at installation (even the FW5K105RC, it has less, but it has), in almost all cards/systems (Tested on OEM Systems). The only thing that I find very different between your cases, is how difficult is to get them working again, in OEM systems, at least in my case (GBC OEM X2, GBA OEM X2 and tested with 3 EZFJr). I had a lot of issues like the ones that all mention but they get fixed just by rebooting (just once), and installing another FW.
What is your test system
@aaaaaa123456789 ?