My inclination is to turn this around and ask "why do you want to use HTTP only for the server?"
Every build I have completed (for the last few years) has been "HTTPS only" on the ArcGIS Server, and HSTS on the web server, which upgrades HTTP requests to HTTPS. That's the approach I'd recommend for everyone. I do not see value in using the HTTP only.
Sorry, not the answer you probably want, but HTTP is dead so it needs to be asked.
There is a way to configure the Web Adaptor form a commandline, but even there you cannot override the ports:
https://enterprise.arcgis.com/en/web-adaptor/latest/install/iis/configure-arcgis-web-adaptor-server....