Upgrade and expansion

Right now we are running a host server that houses our application so that customers who would rather us host it are allowed to do so and it gives them rdp access to it from anywhere. Server had been up and running for a few years now and its currently running 2008 server R2 and has around 175 people that connect to it. Unfortunately when it was setup it was setup as the Active Directory Domain Controller, DNS, File Server, and the Terminal Server. Well I know that need to be split out but because of cost and downtime we will need to do it slowly and was kinda getting my plan in place and wanted to maybe get some opinions and see if I was moving in the right direction. I made a crude drawing of the current layout of what we have and what we can work with currently. I plan on turning all the machine into VMs also. So far I have a host machine running hyper v server 2016 and I have two VMs running on it with Server 2016 installed. The Test Main for instance is the current server running that house everything and where the 175 people are remoting into. Currently we have them remoting into a environment to where they can only see the program which launches when they remote in. I also plan on switching that out for App mode in the new vms. I have added the hyper v server host to the domain along with the 2 server 2016 vms. Now I plan on doing all this without interrupting the Test.Main computer for now as we cant have any downtime with it right now. On the test.main we have the program running off the c drive but all the data is stored on a separate J: drive. I installed remote desktop services on the vm server 2016 1 and it now has the session broker, web access, licence server, and session host roles. I know we shouldn't run all that one one machine but for now it will have to do. I plan on installing a version of our software on the c drive of the vm but it will still access the data off of the test.main 2008 machine this way for the future we can have different vms running different versions of the app but still keep all the data stored in one place. Now the vm server 2016 2 will have a different version of the app installed and have user rdp into it as well as the vm server 2016 1 but the only role I was going to install on the VM Server 2016 2 was the rd session host. I currently have not setup a rd gateway and wasn't sure if I needed it or not some sites say i didn't and wasn't sure if I could just use the connection broker. For now I only plan on a few users on these new vm machines but will add more over time and also plan on upgrading and splitting out the roles on the old main.test machine and also taking the users out. Was just looking for any advice as this is a big project im kinda jumping into.

https://daledemaran.smugmug.com/My-First-Gallery/n-tRkwPP/i-SWSh58Q/A