Now to the question:
I´m going to add internal company info on this private page and i dont really know the security level for the solution.
1. Is this a secure solution?
2. How can i make it as secure as possible with little time effort?
What are your concerns exactly? Like someone could guess the URL and see the content? They can’t, it would be blocked. I don’t think there is any way for anyone to see it unless they are logged in and have proper permissions. So it’s as secure as WordPress itself.