Handling multiple HTTPS servers behind one firewall

Running into a bit of a conundrum here with multiple HTTPS sites on separate servers behind a SonicWall.

Currently Exchange is taking up port 443 with OWA and ECP. Beginning a new RDS instance on Server 2016 and would really prefer if end users didn't have to specify a port number in requests for ease of use and configuration.

I've setup and used WAP and ADFS in the sandbox and would love to go that route. The issue is the domain controller is 2012R2 so I would need to purchase another 2012 R2 license for the sole purpose of running ADFS. I've searched for other solutions and apart from firewalls like Palo Alto and WatchGuard I don't see another good way of accomplishing this.

Do I go against best practice and install ADFS on the PDC? Bite the bullet and purchase another 2012R2 license and then install WAP on the RD Gateway? Or do I make end users get used to specifying the port number? What would you guys do in this situation?

Thanks