Lollipop to Marshmallow (DRK Issue)

Hi :)

I live in Canada so my model # SM-G925W8. I recently bought this phone from someone and noticed that they had it rooted. So I put it back to official stock and it was at 5.0.2 Lollipop and I know there is a more up-to-date Lollipop version and even a Marshmallow version but when I went to check for updates it said I had the latest official update for my phone.

I was kind of disappointed and rooted my device again...

Anyhow after a week went by I went to "fw.updato.com" as suggested by a XDA thread and download a Marshmallow update for my carrier.

I went to supersu and full unrooted
I fully wiped / resetted the phone
I went to odin mode and installed the Marshmallow update

Okay, so anyhow all seemed fine but when I went to boot up my phone I got a DRK (Device Root Key) issue. I figured id just wipe and try again, I even tried downgrading but it wouldn't let me.

I was very scared my phone was hard bricked. So anyhow I decided to flash CF-Auto Root and somehow that managed to fix or bypass the DRK issue and I booted up into Marshmallow! All seemed good in the world. I however noticed my finger print scanner was no longer working saying it was dirty or whatever but it was working with Lollipop.

Anyhow, long story short , I would like to go back to stock but I keep running into the DRK problem. The only thing that seems to let me boot into my phone is the CF-Auto Root and as long as im root I can never have it as Official.

Any suggestions?

Thank you!

~ I would seriously pay for a working solution, I realize I F* up pretty bad somehow. ~