Synchronization tables between more data bases

Hello,

I have a database that is centralized on my clients. In this database, I manage my interventions at my customers (working hours), their computer configuration (computers, server, routers, users, mail, website), offers, correspondence, invoices, documents of the business and many other things. This database is used daily and becomes very very large. Seen that everything revolves around the clients, is it a good thing to separate this database into several small databases? I think yes, already only for maintenance, but how to put the customer tables in all future small databases? One solution would be to create a customer table in each of them and synchronize via the API. So that would mean that I should define in a database the customer table as the main customer table.

Has anyone ever synced tables between multiple databases? Sample code for synchronization? Examples uses with API?

Robert