> Allows me to move directories around, without having to depend on an IDE to keep namespaces in sync.

So... without and IDE, you'd move code around, let the NS be changed due to being placed in a different directory structure, and then fix namespaces on use-sites manually?

> I want all of C#, with terseness and not forcing OOP.

C# does not force OOP on you. You can have a single namespace, single static partial class and spread its members across as many files as you want. So the "ceremony" consists of the following snippet _per file_

    namespace NS;
    static partial class C {
        // Your methods here
    }