Catalina localhost ERR_CONNECTION_REFUSED
The key here is understanding the error message being generated.

When you see a message indicating a connection was refused (ERR_CONNECTION_REFUSED) this means that the server actively rejected the connection attempt.  This is usually a firewall issue where the firewall rejects the packets.

When yo see a message indicating a time out (ERR_CONNECTION_TIMEOUT) this means that your client waited for a response and nothing happened.  This could be due to the service not being up/running, or not installed.

Getting an result back from configtest only checks to see if your httpd.conf file is correctly formatted, not that it's doing what you want it to do.  However, your httpd.conf file has little to do with whether your connection is refused or not other than the IP and/or port it listens on.

Finally, http://localhost and 127.0.0.1 (your hosts file) is a DNS related, not firewall.  An error here can cause your host to be not found but not rejected.


Things to check
So, unfortunately, nothing you've looked at is related to this.  What you have to look for is your firewall.  First, disable your firewall and see if it's working. Next, check your httpd.conf file for the Listen directive and make sure that is set properly.
For Best Results
Unless you're hosting a website on a Mac sever (very rarely done), don't install an AMP stack on your Mac.  Instead, deploy a small VM running the OS that you'll ultimately deploy to.  This way, you can configure the AMP stack exactly for what you'll be using in production and macOS updates won't have any effect on your web server configuration.