Remove 'Secure with an SSL/TLS Certificate' from 'Add a Subdomain'
When adding a subdomain, it takes (at least at my webhoster) 15 min before the changes take effect. If one checked 'Secure the domain with Let's Encrypt' on the page 'Add a Subdomain', an error is thrown because the newly created subdomain is not available in time.
Therefore, I suggest taking this 'feature' off the 'Add a Subdomain' page.
The time required for subdomain to become available depends on a many factors. In your particular case it takes a long time, in lot of other cases it works fast. Hoster can configure the server to wait longer, so, the feature can works even in your case.
That’s why we have no plans to remove this.
But anyway, everyone, please continue voting for this feature if you consider it important. If we’ll see a lot of votes for this request, we’ll reconsider it.
I am an enduser, so I cannot comment on most of your points.
But my web hoster provides Plesk, and I can check 'Secure the domain with Let's Encrypt' when adding a subdomain.