Hi @petr.raunigr,
I confirm Activiz is the best way to get access to the VTK public API from a C# project.
The open-source version is based on VTK 5.8 and is not maintained anymore. The paid version however is based on the latest VTK version and is still developed actively.
The best is probably to give a try to the paid version by requesting a trial version on the Activiz website. Feel free to contact me if you have any issues trying it.
Activiz extensively uses the PInvoke C# technology to call the unmanaged VTK code. It also takes care of managing the VTK objects lifetime to make VTK smart pointers work with the C# garbage collector. In theory you could write similar code to expose your VTK C++ build in your C# project, but that would require a lot of effort.