I've found that activating static batching performs the batching process at build-time, resulting in a massive increase in build size per level (in my case from 400kb to 18MB for a single level).
Is it possible to defer static batching to run-time? Resulting in a smaller compiled project size at the cost of slightly longer level loading?
Note: I am aware of the combine meshes script but it keeps crashing in my tests, I'm still trying to figure this out on my side, so for now I'm just curious about the deferred static batching.
Not that I'm aware of, though it sounds like a reasonable option that could be considered for a future version of Unity, unless there's some technical reason for it other than loading time. The combine meshes script doesn't do what static batching does anyway--for example, static batching works with occlusion culling, whereas the combine meshes script just combines into a single mesh, which doesn't work with occlusion culling.
As previously mentioned, you can use the static batching utility to combine your mesh at runtime. Generally, you’ll want to batch it at build time to reduce the load during runtime. The trade off is build size for performance.