On custom build section you have few example what can be strip:
e.g:
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
Refer to https://www.gargoyle-router.com/phpbb/viewtopic.php?f=7&t=86142、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
Refer to Build for TP-Link TL-WR841N(D) [all versions]3、Remove OPKG & USB function
4、Enable Luci compress
LuCI - Modules - Minify Lua Sources5、Enable data optimization
Remove ipkg/opkg status data files in final images
Strip unnecessary exports from the kernel image
Strip unnecessary functions from libraries
Refer to https://blog.yichyaqc.cn/openwrt-customize-2/6、Find and remove other unrelated parts