As you noted above, you can make snod just make a system image without dependencies.
You can make out/target/product/your-device/system.img , which I suppose will create a system image plus its dependencies, but nothing more.
Just make will probably not do what you want, because the Android build system will create everything, even material that is not addicted - it builds all the other packages in case they are broken into changes in the APIs.
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