Give people only write-permission inside a folder

We have many customer projects that follow a certain fixed structure. 

Lets say my document libraries have all those folders:

- 01 Kickoff

- 02 Process

- 03 Finalization

Thats just an easy example.

My users should now get read/write permission inside those folders, so they shoud be able to add files to "Kickoff", rename files there and even delete stuff etc.

So far so good. But how can I prevent that they change my top-level-structure?

Can I prevent people from renaming the "Kickoff"-Folder somehow but preseving them to read/write inside of Kickoff? With normal SharePoint permissions I do not see any way... but there must be a trick before I write my own SPFx plugin.