[ZTE Z7, Z7 Max, Z7 Mini] iggarpe:
I spent the whole evening yesterday banging my head against my Z7. This is what worked for me, and bear in mind I repeated the whole process again at the end just to be sure.

Prerequisites:

1- Nubia Updater, 2.02 full ROM, 2.03 full ROM, 2.05 update (all found in this thread's first messsage).

2- 3.88 update (here: https://mega.nz/#F!R9VGiIyb!eYNn-ySQAxNZgF2FP2xtOA).

3- GAPPS from opengapps.org (download for ARM/5.1/micro).

4- This TWRP version: https://mega.nz/#!ctkxlabD!rPlcL4mrCz8oNwZ_JqmR1Or2XXYThAQcX-38D_ABNa0.

5- adb and fastboot command line tools plus Android drivers (installer here: https://xdaforums.com/showthread.php?t=2588979).

6- Phone drivers. When you first connect it via USB a drive will show up with a driver installation program.

7- Qualcomm's QPST tool (OPTIONAL).

[A] - Make a QCN backup. This is optional but will let you restore your IMEI and other important info if for some reason you screw up your phone.

1- Dial *#7678# and enable both ports. Connect phone. Wait for drivers to be installed.

2- Create C:\Temp directory.

3- Launch QFIL application (in Qualcomm's QPST) go to Tools / QCN Backup Restore and make the backup. BE PATIENT, IT TAKES ~1m.

4- When it finishes you should have a ~384KB file in C:\Temp. Put it somewhere safe.

- Update to 2.05 following this thread's first post instructions. I'll reiterate:

1- Dial *#7678#, enable both ports.

2- Uncompress 2.02 zip file.

3- Launch Nubia Updater.

4- Select 2.02 directory (not the zip file, the directory created when you uncompressed it).

5- Hit the chinese "go" button.

6- At about 92% installation will get stuck and you'll need to manually power on the phone, then it'll proceed to 100%

7- Repeat above steps for 2.03.

8- Copy 2.05 update to phone internal storage.

9- Reboot to recovery (power cycle while holding volume up pressed or command line "adb reboot recovery").

10- Select language then install 2.05 zip update.

[C] - Install 3.88

1- Copy 3.88 update to phone internal storage.

2- Reboot to recovery (power cycle while holding volume up pressed or command line "adb reboot recovery").

3- Select language then install 3.88 zip update.

[D] - Install GAPPS

1- Copy GAPPS zip file to internal storage.

2- Enable developer options (see note 2 below) and then enable OEM unlock. You'll see the switch go on then off by itself. No worries.

3- Dial *#7678# and enable both ports.

4- Reboot to bootloader (power cycle while holding volume down pressed or command line "adb reboot bootloader").

5- Make sure your phone is shown with command line "fastboot devices".

6- Unlock the bootloader with "fastboot oem unlock".

7- Launch TWRM in RAM with command line "fastboot boot TWRP_nx506j_511_v388.img". This will boot TWRP only once. IT WILL NOT FLASH IT (see note below).

8- Disconnect USB cable as soon as the above command finishes uploading the TWRP image.

9- Phone will seem stuck at the Nubia logo. Leave it alone. DON'T TOUCH ANYTHING, just disconnect USB cable and leave it alone.

10- After ~10m the screen will turn off. Press power button briefly to power it up and you'll see you're in TWRP.

11- Install GAPPS zip file.

Note 1: instead of "fastboot boot" you could use "fastboot flash recovery" to flash the TWRP image. However (a) the 10m wait makes it IMO rather useless. I'd rather have the stock recovery that works just file. If you need to install non signed packages just launch TWRM image in RAM again, and (b) even if you flash it I seem to recall that it gets overwritten later at some point with the stock recovery.

Note 2: go to phone info, tap multiple times on "compilation number", then get out --> "More" --> "Developer options".

You should be able to install SuperSU zip using the same method you used to install GAPPS, but I haven't tried it.

Fun fact: the Z7 has a 2K screen with a nice feature: internal LCD image RAM so the CPU does not need to keep constantly refreshing the screen when image is static (this saves power in a normally power-hungry 2K screen). Apparently the TWRP recovery image does not properly initialize screen refresh so what you are stuck with is the bootloader image, which is being shown autonomously by the display in lack of proper refresh from the CPU, thanks to the LCD internal RAM. When the screen is powered down and then powered up again it seems the refresh of the screen is properly initialized and now you can see TWRP display and menus.

EDIT: I did indeed end up losing my IMEI at some point, most likely not as a result of the above descrived procedure but of some other messing I did. In any case, once I had 3.88 with GAPPS up and running, it was just a simple matter of launching QFIL again and restoring the QCN from the backup I made first. So my advice is as soon as you get your hands on your Qualcomm based phone, make a QCN backup, it might end up saving your day

EDIT: as kindly pointed out below, you can make TWRP more usable by setting the standby timer to 15 seconds, so you don't need to wait so long for standby/poweron workaround.

EDIT: added the bootloader unlock step.