The highest domain functional level even with 2019 domain controllers is 2016. As to application compatibility the developer will be your best resource for this information.
The two prerequisites to introducing the first 2019 domain controller are that domain functional level needs to be 2008 or higher and older sysvol FRS replication needs to have been migrated to DFSR
I'd use dcdiag / repadmin tools to verify health
correcting all errors found before starting
any operations. Then stand up the new 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 again verify health, when all is good you can decommission / demote old one.
--please don't forget to Accept as answer if the reply is helpful--