Great!
Is there a good way to do that Nandroid backup without root?
I have found a TWRP guide that says you need root to use TWRP, but in the guide to rooting it says you need a custom recovery first, which I a TWRP recovery, I guess.
No no, something is wrong with the guide.
Get this TWRP -
https://dl.twrp.me/m8/twrp-2.8.6.1-m8.img.html
Put your phone into FASTBOOT USB mode, just like when you got your token.
Copy/move the TWRP download to the folder with your fastboot.
Command window say -
fastboot flash recovery twrp-2.8.6.1-m8.img
Wait for OKAY and something like 16 MB transfered message on pc.
On phone, navigate to hboot, then navigate to RECOVERY.
TWRP will appear.
If not, reboot into bootloader and go to recovery again.
If TWRP detects that you are not rooted and offers to do it for you, say NO. Don't let it do that.
Main menu, Backup, accept the defaults (change internal or external sd card as you like) - and go.
Now you have nandroid backup of your stock unrooted phone.
Reboot phone.
Move ARHD rom zip file to phone storage.
Boot into TWRP. Main menu - Install. Navigate, find the ARHD rom zip. Go.
(Check the XDA thread I linked for exact install instructions. Wipe before installing if they say to from TWRP. Some roms will wipe before the install themselves, I don't remember with ARHD.)
Reboot when done, get everything set up and right so you have a working phone.
Boot into TWRP, make another nandroid backup.
Reboot phone, copy your TWRP folder to your pc for safekeeping. If you run out of phone space you can delete nandroid backups on your phone if you already save them to your pc.
Now - try out any of your desired mods.
If something gets bad, TWRP, Restore, get last good nandroid backup, move forward try again.
Make a nandroid backup for every successful major modification.
Pro tip - you can get an inexpensive OTG USB cable, plug in to your phone with USB stick formatted as FAT32 and do all of your nandroid backup and restore from there, no worries about your limited storage.