I badly need your clever advice!
The thing is I am trying to extend on-premises active directory to Azure. And one of my coworkers mentioned that if it’s over 100,000 items then you’ll want to install to full SQL server and installer really is not very well designed for such installations. Therefore I moved to custom installer.
On the first look, the process for doing this should be quite straightforward as it’s just an installing the Azure Active Directory Connect tool onto a server and then, creating the domain in the Azure portal and then waiting for Azure AD connect to Sync.
Troubles came when I did well through the 99% of the way and Synchronization process was finished. I just wasn’t able to login to the SQL server.
The only heads up is the small red bar which appears under service account name/service account password.
Azure AD connect custom installer is a tough enemy to fight with. And as you truly mentioned the errors is not clear and from time to time I have a filing that MS just don't want to waste a code on that bunch of errors.
Some time ago I was tasked with the pretty same task as is you are trying to accomplish ( to sync on-premise AD and Azure, and there were more than 200k items, so I have started to dig in the direction of custom installer)
I have been digging the web and outcome was quite poor, the only this article https://www.starwindsoftware.com/blog/fighting-azure-ad-connects-custom-installer. Can be helpful for you.
By default Azure AD Connect uses a virtual service account for the synchronization services to use. If you use a remote SQL server or use a proxy that requires authentication, you need to use a managed service account or use a service account in the domain and know the password. In those cases, enter the account to use. Make sure the user running the installation is an SA in SQL so a login for the service account can be created.
More information about service account, please refer to this link.