After setting up the reverse proxy for Apache per the current instructions, I received the error: "AH01144: No protocol handler was valid for the URL / (scheme 'http')". Installing proxy_http and proxy_balancer modules in addition to those listed fixed this issue.
a2enmod command does not include the redundant '_module' suffix.
Let's Encrypt was not able to validate certificates either automatically or manually as ABS does not respond to ACME challenges directly. Editing the virtual environment configuration to ignore proxy requests to .well-known and serving that directly from Apache allowed LE to validate the challenge instead.
