My purpose is to write the device identity authentication information into my customized Omega2+ device. I hope this part can be firmly stored in the device (retained even after system restore).

I confirm writing uboot variable is available on uboot environment, refer by https://community.onion.io/topic/2272/what-does-this-message-mean_=1767832343343.

But modify uboot variable only on boot environment is not good idea for me, fw_setenv is available on system environment? Permission Denied for /dev/mtd1 When Executing fw_setenv.

are you building your own firmware for your device? If so, you can just edit the device tree target/linux/ramips/dts/mt7628an_onion_omega2.dtsi around line 120 and remove or comment out the read-only:

partition@30000 {
	label = "u-boot-env";
	reg = <0x30000 0x10000>;
	read-only;
};

And then rebuild the firmware.

That's what I have in my firmware for many years now for similar purposes.