Is a company internally developed CMS a good idea? Its build as a Laravel Package.

Almost certainly no. Why would you when there's so many free ones out there where all the hard work, updates etc. are done for you?

There is 1 use case: the company has a lot of resources (time, money, and skilled developers and sys admins) and the managers feel that at least one of their specific demands is not met by one of the thousands of existing options and they feel entitled to those resources.

Been there btw.

Otherwise no.

Yes and no. A specialized tool that uses your companies lingo and is designed to especially tackle your use cases pays off in the long run by needing less onboarding for new staff using it than having a more general solution. Also you control what it does so the situation of an update breaking your work flow and forcing a migration of all your data will not happen.

On the other hand you put the burden on you to develop a system that will eat up dev, admin, test and hardware capacity because you can't just file a ticket with another company.