So the current problem is that mounting the docker data_root on an ext4 drive doesn't work because CONFIG_KERNEL_EXT4_FS_SECURITY isn't set in the OpenWrt kernel by default?
The problem with setting a kernel option on the package level or compiling your own custom kernel generally means that you need to compile and install updates yourself one way or another.
So it sounds like the only path forward would be to set CONFIG_KERNEL_EXT4_FS_SECURITY in the kernel by default or it sounds like mounting to a different filesystem like BTRFS works too.
Though, BTRFS specifically, you might want to check that you don't have snapshots of your docker snapshots or you'll fill your drive quickly!