Actually, it’s probably not possible with sysfs, otherwise I would have recommended sudo chattr +i /sys/class/leds/standby/brightness
Instead, you can use custom wake / sleep scripts for now. See [HOWTO] Vero 4K / 4K+ / Vero V: Performing actions programatically on Standby and Wakeup for an example. You can run any commands you want when the device is suspended / woken up.