Making minimized firmware can properly enable/disable the following options(With BuildRoot):

1、Set Target Images Block size to 1024
Target Images - squashfs - Block size - 1024

2、Disable kernel debug and wireless debug
Global build settings - Kernel Build Options - everything
Kernel modules - Wireless Drivers - kmod-ath9k - Support for Ubiquiti Unify Outdoor+
Kernel modules - Wireless Drivers - kmod-mac80211 - Export mac80211 internals in DebugFS
Kernel modules - Wireless Drivers - kmod-ath - Force Atheros drivers to respect the user's regdomain settings, Enable DFS support

3、Remove OPKG & USB function

4、Enable Luci compress
LuCI - Modules - Minify Lua Sources

5、Enable data optimization
Remove ipkg/opkg status data files in final images
Strip unnecessary exports from the kernel image
Strip unnecessary functions from libraries

6、Find and remove other unrelated parts