I set CONFIG_ALL_KMODS=y to a new config and built yet the resulting image is the same size as my customized one containing a subset of all modules. I would like to have all possible modules installed to the image to /lib/modules/$(uname -r), not as kernel built ins.
Start with a minimum skeleton (init-) config, just target/ subtarget/ device and the non-default settings you need (including ALL_KMODS), make defconfig will expand it for you.
I tired that but the resulting image did not contain all 800+ modules.