Migrating Domain Controller from Windows 2003 SBS

While it would be possible to migrate directly from 2003 to 2016 (but not 2019, as it requires Windows Server 2008 functional level), I recall Small Business Server (SBS) being quite a different beast, having some adjustments that may cause some unnecessary burden. With SBS licensing you were only allowed to have one Windows Server in your environment, which is against all other recommendations of having at least two domain controllers, and that might also be a cause why your migration is not going as expected.

Your current environment and approach have at least three challenges:

1. Moving from 2003 to 2016 is quite a leap.
2. Moving out from SBS.
3. You are using a .local domain, which is not recommended.

Unless it's absolutely impossible to create a new Windows AD from scratch and migrate the data rather than the current AD domain, I would suggest installing a fresh AD with a publicly registered ad.example.com subdomain. As SBS was designed for small companies with 25 to 50 workstations, it shouldn't be an out of question task in a small environment like that.