For a massive, multi-million line CAD/BIM project, it makes strictly economic sense to focus on Windows first. Expanding to macOS and Linux drastically increases development, QA, and support costs, which outweighs the initial revenue potential of non-Windows platforms in the professional CAD/BIM sector.
## Why Focus on Windows?

* Market Dominance: Windows holds roughly $90\%+$ of the enterprise, engineering, and architecture market share.
* Hardware Standardization: Professional CAD workflows rely heavily on workstation-grade NVIDIA GPUs and specific hardware drivers. Windows offers the most stable and ubiquitous ecosystem for this hardware.
* Legacy & File Dependencies: You will likely rely on established C/C++ libraries, rendering engines (like HOOPS or Redsdk), and CAD kernels (such as ACIS or Parasolid). These have deep, native Windows support; porting them to other OSs is incredibly difficult and expensive.

## The True Cost of Cross-Platform Support

* The "Write Once, Run Anywhere" Myth: For complex desktop software handling 3D operations and BIM, you cannot use cross-platform frameworks (like Electron or Java) without killing performance. You will need to write and maintain three separate native codebases (or use an intermediate heavy abstraction layer).
* Graphics API Bottlenecks: You will likely use DirectX 12 on Windows to handle millions of polygons. Moving to macOS means supporting Metal, and Linux means supporting Vulkan or OpenGL. This necessitates maintaining entirely different graphic pipelines.
* Triple the Testing: Quality Assurance (QA) becomes a nightmare. Bugs in 3D rendering and memory management manifest differently on NVIDIA, AMD, and Intel GPUs across hundreds of OS configurations.
* User Support: Professional users expect immediate, enterprise-level support. Diagnosing complex crashes, memory leaks, and API failures across three entirely different operating system environments requires specialized teams for each.

## How to Approach Expansion

   1. Launch on Windows: Get your Minimum Viable Product (MVP), core 3D operations, and API stable for the professional AEC (Architecture, Engineering, and Construction) market.
   2. Consider Linux (for Server/Render Nodes): If your CAD program includes a server-side component or rendering engine, you should absolutely build the core in a cross-platform language (like C++) so those headless backend services can run cost-effectively on Linux servers.
   3. Explore the Web / Cloud (Future-Proofing): Rather than building native Mac/Linux UI ports, consider a WebAssembly (Wasm) or thin-client architecture. Technologies allow you to process heavy 3D/BIM data on the cloud and stream it natively to any modern browser on any OS.

If you are planning to build this as a viable commercial product, you can significantly reduce your financial risk by targeting your ideal market first.
Could you tell me more about who your target users are (e.g., architects, mechanical engineers) and what foundational CAD kernel or language you are planning to use? That can help refine this deployment strategy.