A. yes, sounds correct
B. you wouldn't need to wait but you can check if cleanup is needed prior to adding new one https://video2.skills-academy.com/en-us/windows-server/identity/ad-ds/deploy/ad-ds-metadata-cleanup. Some general steps
I'd use dcdiag / repadmin tools to verify health correcting all errors found
before starting any
operations. Then stand up the new 2012, 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.
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