Two PDCs, two ADs, two domains - how to replicate one domain/AD to the other?

Here's the history:

SERVER2 was a 2016 Essentials Edition server, standalone with no other DCs. The OS became corrupted in a few areas, and so a decision was made to replace it. A standalone clean install wasn't an option, as applications running on member servers rely heavily on AD user SIDs.

So a second DC was introduced, SERVER3, and the domain/AD/DNS/PDC/fsmo were replicated from SERVER2 to SERVER3. Metadata cleanup was performed on SERVER3 to rid it of any old references to SERVER2. SERVER2 has now been taken permanently offline.

A brand new SERVER2 Essentials Edition has been configured, and it has its own domain/AD/DNS/PDC/fsmo. The display names of the two domains are the same, but the underlying ADs are of course different.

How do I make the new SERVER2 a BDC for SERVER3, replicate everything from SERVER3 to the new SERVER2, and then promote the new SERVER2 to be PDC?

I had some expert assistance to get this far, but unfortunately the tech has been called away. I'm now on my own, mid-project.

Please advise.

--EDIT--

I found this guidance, but it doesn't seem to take into account that I have two PDCs on separate existing domains.

Solution 1:

Windows Server Essentials requires being a Domain Controller, but it can be added as replica DC to an existing domain, as long as you are doing it for migration purposes (i.e., you as long as you are going to remove the existing DC):

https://docs.microsoft.com/en-us/windows-server-essentials/migrate/migrate-from-previous-versions-to-windows-server-essentials-or-windows-server-essentials-experience

I'm not really experienced with this heavily-locked-down edition of Windows, but if you go digging in the documentation, it should be possible to replace your existing server with a new Windows Server Essentials one.

However, you'll definitely have to rebuild your new server from scratch, because your current situation (each server hosting its own Active Directory domain) is a dead end and there is no way to "merge" them.