Configure URL Masking & Proxy Redirection

I am looking for an experienced server administrator or web developer to configure URL masking and proxy redirection on my Apache server. I use AA panel. The primary goal is to serve content from smartifyerp.online while keeping demo.smartifyerp.com as the visible URL. Both smartifyerp.com and demo.smartifyerp.com are currently hosted on the same website environment without separate VirtualHost entries.

Objectives:

Configure .htaccess to proxy requests from demo.smartifyerp.com to display content from smartifyerp.online, while preserving the demo.smartifyerp.com URL in the user’s browser.
Ensure all assets (CSS, JS, images, etc.) and dynamic routing (through index.php) function seamlessly when accessed via demo.smartifyerp.com.
Prevent double or redundant slashes in URLs and resolve any resulting 500 internal server errors.
Ensure that all necessary Apache modules (like mod_proxy, mod_rewrite, etc.) are configured and active.
Requirements:

Expertise in Apache configuration, including .htaccess rewriting and URL masking.
Experience with proxy configuration using Apache [P] flag for URL masking.
Strong troubleshooting skills to resolve internal server errors and any issues with SSL.
Clear documentation on any changes made for future reference.