I made it a wiki. You are welcome to update the OP to better detail everything. Since you have expertise here, I do hope that you’ll watch this topic so that you can help others who want to follow in your path.
The fact remains that no one who is a regular contributor here uses Cloud Panel, and the person who created the guide isn’t participating in supporting it. Supporting just the standard install, where there is very little that can go wrong, is fairly time consuming.
Assuming no one objects, I’ll incorporate my suggestions when I have time to make sure they flow properly (and work). I know at least running discourse-setup with --skip-connection-test will be required.
I think this is probably unfair, they have been updating the guide based on feedback and probably other people have just been getting to questions first.
At the time I didn’t understand that it was also necessary to change other points of the app.yml file such as mail server and other things, since the guide here was made for experts, only the most obvious things were indicated for experts and not also things for inexperienced people like me. Then I found difficulty with the mail configuration as my provider has port 587 in TLS.
Besides following your guide, I let all other Cloudflare settings by default and only modified those you mention in the guide.
I don’t know anything about advanced installs and the error might not be related to your guide, unless there’s a missing bit somewhere – but another user had it working, so I guess it’s more likely on my end.