You can do it using direct gpio access.
Some leds can be controlled using /sys/devices/platform/gpio-leds (medium-level, but you can override driver settings there), and all of them can be directly controlled using /sys/class/leds.
You can read this article to understand the principle - https://medium.com/openwrt-iot/lede-openwrt-disabling-your-leds-5eaf51735ff1
LAN leds are also can be controlled trough options in /etc/config/networks, see example here - [solved] Disable LAN an WAN leds, but keeping WLAN leds
So, just create a script filling led pins with zero and run it at startup and change network config. Should be enough.