Did you try manually installing kernel modules taken from bin/ar71xx/packages/ after the build? This way you won't need to reflash the whole firmware and just install the modules using OPKG.
I installed the following packages taken from the GPL build (With correct filename applied in RUT9XX_R_GPL/package/kernel/linux/modules/crypto.mk) directly on the router (Transfer using scp, install using opkg):
And it seems that ext4 mounts successfully.
Note: I had to use --force-depends option in opkg.
I'll report the issue to RnD regarding the incorrect file name specified (crc32c_generic.ko) for kmod-crypto-crc32c to build correctly
but I cannot say if the option will be enabled by default, most probably it will be disabled by default and up to the user if he want's to build with it or not.