Some years ago we got a new customer with a lot of similar TYPO3 installations. All had their own Git repository. We decided to migrate this setup to composer and split the existing TYPO3 extensions out of the repositories, into their own. This year we noticed that was a bad idea, and we will migrate all installation into a single Git repository, together with all extensions.
I'll try to explain the initial idea, why we think this was a bad idea, and how we create the new mono repository.