Lots of trouble unrooting M8...

I've been having a hell of a time trying to unroot my HTC One M8... It was an AT&T one I converted to Skydragon GPE, S-off. All the resources I try to use keep on failing somehow—anyone have suggestions? There's a million different tutorials on how to do it and none of them seem to work for me, getting really frustrated...

(I would link the below guides I've been trying to follow but I'm too new a user so it won't let me):

- The Unlockr guide: The Hassoon toolkit no longer exists.
- DroidViews guide: Everything's good until I try to restore the RUU image, then the executable fails with no explanation. Same when flashing the RUU from HTC's website.
- Restoring from a Nandroid backup in TWRP: Also never makes it through the entire process, I'd have to rerun it to see what the particular error was.

Ughhh is there any definitive set of steps I can take to actually restore to default settings? Preferably with the tampered flags not tripped, but honestly I don't even care at this point... Do I need to flash some stock AT&T HTC One M8 firmware or something before doing this all? Where might I be going wrong? I know it's vague but I'd love a solid walkthrough.

Thanks everyone :)