Check that DFL is at 2008 R2 level,, also FRS needs to be migrated to DFSR before adding 2019 domain controllers.
https://techcommunity.microsoft.com/t5/Storage-at-Microsoft/Streamlined-Migration-of-FRS-to-DFSR-SYS...
To the domain controller migration general steps below;
I'd use dcdiag / repadmin tools to verify health correcting all errors found before starting. Then stand up the new Server 2019, patch it fully, license it, join existing domain, add active directory domain services, promote it also making it a GC (recommended), transfer FSMO roles over (optional), transfer pdc emulator role (optional), use dcdiag / repadmin tools to verify health, when all is good you can decommission / demote old one, move on to next one.
