XT 11 speed derailleur (RD-M8000-GS) rubbing 46T cog with b-screw all the way in? I've broken my rear derailleur so I've decided to upgrade from my 2x10 to a 1x setup. For that, I've initially bought:

Deore XT 11 speed derailleur RD-M8000-GS (I've read it can work with 10s, but indexing is not the problem now).
MSC 10 speed 11-46 cassette (I may change this by an 11 speed, same range. See below*).
30T chainring, a chain, and I'm keeping my old 10s Deore shifter (guess these aren't relevant).

My problem is regarding range. It looks like the derailleur wont climb to the 46T cog, even though:

By the spec, it supports 11-46 in 1x setup.
The b-screw is all the way in.
My hanger looks like a direct mount (goes back and outside) and the derailleur is installed as so (no extra link).

* I understand a 10s cassette is not the same as an 11s, but I guess that the biggest cog (46T) is the only one that is in the same position (flush to the base of the hub), so I fear if I buy an 11s cassette (and an 11s shifter) I'll have the same issue.
What are your thoughts? Thanks in advance!
As the comments suggest, you need to install the chain to adjust the rear mech. You can test this yourself by manouvering the shifter to mimic having a chain and you should notice that the mech cage can move up and down.
The proper intaliation process is:

Install derailleur with b screw half way on the hanger
Install chain, length calculated using the manufacturer's instruction
Adjust b screw according to manufaturer's instructions
Fine tune the shifter/ cable
Re-check the b screw as the fine tuning of the cable can change it

On a side note, I tried this same setup, but with xtr 11s mech and xt 10s shifter, and the middle gears where always off and a little jumpy. So I just bought the 11 shifter and I wouldn't recomment mixing it.
Whether the mech is direct mount or not, shouldn't affect this, as long as the mount is not bent or damaged.