[Q] Stock, Unrooted D850 On 21r -- Easiest Way To Get TWRP?

I have a new D850 that I want to flash my backup from my other D850 onto. The new one is:

Code:

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Android version
5.0.1

Baseband version
MPSS.DI.2.0.1.C1.13-00058-M8974AAAAANPZM-1

Kernel version
3.4.0

Build number
LRX21Y

Software version
D85021r

Security software version
MDF v1.1 Release 3

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So my goal is to get TWRP onto it, ideally 2.7.8.0 as that's the one on my old phone I made the backup from, but any version will do.

What are my options? My understanding is that:

I need root to install TWRP
D850 10F and above cannot be rooted currently
However, it is possible to downgrade... and then root?

Do I really need to go through all of that just to flash my backup? Is there a simpler way to get TWRP loaded?

If not... can someone explain the downgrade process? I see hyelton's guide, but I'm so very confused as to which of the methods (LGUP, KDZ, TOT) apply to my situation – D850, downgrading from 10R to something rootable – and which files I'd actually need... :S

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