Upgrading from 10.12.6 to 10.15.6

File Sharing (still with inheritance issues) and Caching are no longer part of what Apple ironically call "macOS Server". Both are part of the standard Client OS. An overview of what's on offer is here:

https://www.apple.com/uk/macos/server/features/

Not much is there? Basically a baseline (read basic) MDM - Profile Manager, Open Directory (with much less operability than before) and Xsan. I don't see Time Machine Server (such as it was) mentioned either. For your needs I doubt if purchasing and using macOS Server is of any benefit? If it was me I would either stay with what you have, or purchase the new hardware and use the standard OS with the full understanding that some aspects of File Sharing may/will give you problems.

Ultimately it's your choice as only you are best placed as to what serves you best.

HTH? Tony

Michael Ginsberg wrote:

My question is if I go directly to Catalina 10.15.5 will all my users and file sharing settings be intact after the upgrade?  

You would need to re-add your Users in System Preferences/Users & Groups as Sharing Only. And re-do your file sharing settings in System Preferences/Sharing. For Time Machine you would stay in Sharing, select the folder for TM from your Shared Folders, right click it, select Advanced Options and check Share as a Time Machine destination.

If you don't uninstall your macOS Server software BEFORE you transition to Catalina, you may be prompted to upgrade to macOS Server for Catalina in order to use it. Personally, I would uninstall macOS Server before upgrading to Catalina. Because Catalina's macOS Server is worthless for your needs.