Tracking multiple websites for clients

I am a newbie to piwik, so pardon my ignorance.
I would like some advice on how to accomplish what I want to do.

I have multiple clients who will install/copy-paste javascript snippet into their website and all the tracking data would come to a central common database. I think piwik already does this.

I want to customize one area, the UI. The clients will login to my website and from their profile add their website through a custom UI we have created (not through piwik site). The rest of the process will be handled by piwik to add it to the central DB.

How can this be done? Any specific links to tutorials/articles appreciated. I am sure many of you have already done this. I would like to know your thoughts. Thanks in advanced for the answers and help.

You should use the Piwik API to create websites and users automatically: Piwik Analytics API - Analytics Platform - Matomo