Any good AD software?

I've got a situation where I'm at a company with 2 domains.

1 does their GPO, AD (For Windows logins) and is their old AD that they want to move from.
The other is their new AD that is only used for domain names.

At the moment, I have to create a new user in the old AD as well as in the new AD. This is really annoying and I'd love to automate the process some. The other annoying aspect is that the new AD also uses Azure AD to sync to O365.

I guess I'm looking to be able to have the old AD, sync to the new one for user/password info, and then that syncs using Azure to O365, so people can use 1 password for Logins and for Office.

Anything that can assist would be outstanding. The eventual plan is to do a domain migration, but that's a long process and I'm just looking to save a little time right now.

Why not just migrate everything off the old domain?

I have used Binary tree in the past for AD migrations, its a beast to setup but works well if you have the skills.

That's the eventual plan though. But it's a long time out.