According to Microsoft, Exchange 2013 (highest CU) isn't compatible with a domain controller level of 2019. At least if I'm reading this matrix correctly:
With a 2012R2 domain currently, if I build a 2019 server and promote it to domain controller. Will this cause issues immediately or only if the domain/forest functional levels are upped?
I wouldn't immediately raise the domain/forest functional levels. If this works, I would be on 2012R2 domain/forest functional with 2019 domain controllers in the hybrid domain setup, until our Exchange environment is migrated and then we can up our domain/forest functional levels.
I'm thinking I will have to either migrate Exchange 2013 to either 2019/Exchange Online first, then we can build our new domain controllers and continue the migration.