If this is for demo, even more DNS is not necessary. Just code your web server to serve challenge for that particular Uri, and you are done. I do not think so that you need wildcard cert for the demo. Just fixed subdomain is fine.
Also you do not have to wait for DNS to propagate with HTTP method.
I think we do not understand each other. Look at this: https://letsencrypt.org/docs/challenge-types/#http-01-challe...
If this is for demo, even more DNS is not necessary. Just code your web server to serve challenge for that particular Uri, and you are done. I do not think so that you need wildcard cert for the demo. Just fixed subdomain is fine. Also you do not have to wait for DNS to propagate with HTTP method.