HI
1.can you enter winver in command prompt on all cals needed server and look the os version ?[for example server 2016 standard]
if 2 server(act as DC/file server) and 1 server(act as DHCP) are the same OS version(example server 2016 standard),we can purchase server 2016/2019 standard cals for device or user.
2"If the environment has the following, is it better to go with the device CAL or user CAL?"
in modern time ,one user will have more than one device(like ipad,iphone,linux etc) to connect to server for test purpose(or other purpose).
so when the number of devices is more than the number of users ,i think purchasing windows server per user cal will be better.
3.for purchasing windows server 2019 cals ,we can contact volume license center support or windows server retail store support.
https://www.microsoft.com/en-us/cloud-platform/windows-server-pricing
Best Regards,
Anne