So last night I hopped to (Non)Guix on my PC, which took me 4 hours total and had plenty of pitfalls.
I made a lengthy writeup with lots of links to other posts that helped me through the process on my blog.
Link: đź’» (Non)Guix - The Uninstallable
Hope this helps someone
More Guix content, nice 
Looks like we had basically the same experience installing NonGuix 
The manual modification to add the non free parts is really something… and we all had to do it. Not sure if maybe someone has tried to work on an easier way to install it though.
Btw, on your website the button to go to the homepage was not always there or very small 
Pretty much all of it already covered in nonguix readme and guix manual. But many people refuce to simply follow instruction, they want to recklesy dive into things. I know it by myself
Two more things:
• you can check substitutes aviability through guix weather command
• consider adding nonguix substitutes to your config.scm, so no need to add it manualy with --substitute-urls= everytime
Update regarding the “no space left on device”, it appears I overlooked to run herd start cow-store /mnt.
As for the docs, I’m sure they are great once you’re settled in and need to check something for reference, but due to policy they do not include nongnu instructions in mainline guix docs.
The nonguix readme is good, certainly sufficient for someone who knows how to install standard guix already, but incomplete for a new user.
The friendly community is very helpful 
Thank you very much for the writeup.
I had quite a frustrating experience and worked it out using a combination of what you did, together with the info from Nonguix Installation Guide - System Crafters
The nonguix installation guide made me run into issues right from the start as my wifi had to be unblocked from the bios, which i had not figured out at that time and went with the visual installer, until it showed the final config.scm for review. Then i switched over to a different tty and followed the steps from the installation section at Nonguix Installation Guide - System Crafters up until “Update system configuration”.
So:
herd start cow-store /mnt
git clone https://github.com/daviwil/dotfiles
mkdir -p ~/.config/guix
# the path inside the dotfiles gitrepo has changed
cp dotfiles/.files/.config/guix/channels.scm ~/.config/guix
guix pull
hash guix
Now i had my sources all set up, went on to edit the config.scm the way you described, extending the use-module and operating-system calls.
Then i used your command to install the system, rebooted, logged in as root without password, set the passwords for root and my user again, now everything seems stable.