I'm getting ready to promote our current domain from a Server 2008 R2 server to Server 2016. All dcdiag checks and replmon checks are good and looks to be ready. However I've been doing reading on DFSR and realized we are still on FRS.
My question is do I need to migrate to DFSR (this is recommended over FRS) now, or is this done when the new Domain Controller is promoted? Or if I should migrate AFTER I promote my domain to 2016?
Yes. You should have migrated to DFSR years ago.
Migrating to DFSR is more important than upgrading to Server 2016. Prioritize the DFSR migration.
Migration guide: https://blogs.technet.microsoft.com/filecab/2014/06/25/streamlined-migration-of-frs-to-dfsr-sysvol/
FRS is removed from RS3 aka Server 1709. Server 2016 RS1 aka 1607 will be the last version of Windows Server to include FRS.