This will be simpler than Windows. You'll need XCode.

• Fork the source code of Lipika IME, open it in XCode, play with it, understand how it works
• Integrate GoVarnam into it
  • Build for Mac. The output will be a .dynlib or .so. This will be easy. Just do make nix. Maybe add Makefile rules specifically for Mac
  • GoVarnam functions can be called from Swift https://stackoverflow.com/q/34669958/1372424
  • GoVarnam API usage, example is mentioned in the Windows task Varnam IME for Windows #7
• Automate Mac build via CI

An early installer is available here : https://varnamproject.com/download/mac/ . It's quite stable

Repo: https://github.com/varnamproject/varnam-macOS