Ok, so then there's no chance. The 2016 will need to be rebuilt from scratch. I don't remember which one has the roles but roles need to be moved to 2008, (seize if necessary)
https://learn.microsoft.com/en-us/troubleshoot/windows-server/identity/transfer-or-seize-fsmo-roles-in-ad-ds
then take the 2016 offline, and do clean up to remove the 2016 remnants from active directory.
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/deploy/ad-ds-metadata-cleanup
https://techcommunity.microsoft.com/t5/itops-talk-blog/step-by-step-manually-removing-a-domain-controller-server/ba-p/280564
then make sure system and FRS Replication event logs are free of errors before trying again.
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