Hi,
For your questions:
If you want to keep your 2012 DCs in the domain, the function level can't be raised to 2016.
If you want to raise the function level to 2016 , the 2012 DCs must be demoted (after the new DC was added) .
To add the 2019 DCS:
The minimum requirements to add a Windows Server 2019 Domain Controller is a Windows Server 2008 functional level. The domain also has to use DFS-R as the engine to replicate SYSVOL.
Since you have 2012 DCs in the domain, make sure the fonctional is 2012 (or raise it to 2012 ), and confirm the DFSR is used for the sysvol replicaiton.
You can confirm the DFSR is used by the command dfsrmig.exe /getglobalstate
If the Result: 3 (ELIMINATED) the DFSR is used for the replication.
If not, FRS should be migrated to DFSR:
https://learn.microsoft.com/en-us/windows-server/storage/dfs-replication/migrate-sysvol-to-dfsr
After the 2 requirements meet, , we should also make sure everything is working well, following command for your reference :
Dcdiag /v >c:\dcdiag1.log
Repadmin /showrepl >C:\repl.txt
Repadmin /showreps *
Then the new DC can be added , following for your reference:
https://www.petenetlive.com/KB/Article/0001262(third-party link)
Last, you can consider to keep the old DC or raise the functional level.
This response contains a third-party link. We provide this link for easy reference. Microsoft cannot guarantee the validity of any information and content in this link.
Best Regards,