This should change the brightness level of a single external display connected to an M1 MBP or MBA relative to the current setting using DDC/CI. Reportedly does not work on M1 Mini.

chbrt.m

@import Darwin;
@import Foundation;
@import IOKit;

/*******

This should change the brightness level of a single external display connected to an M1 MBP or MBA relative to the current setting using DDC/CI. Reportedly does not work on M1 Mini.
Credits to @tao-j and @alin23

Compile:

clang -fmodules -o chbrt chbrt.m

To increase brightness by 10% enter:

./chbrt 10

To decrease brightness by 10% enter:

./chbrt -10

*******/

typedef CFTypeRef IOAVServiceRef;
extern IOAVServiceRef IOAVServiceCreate(CFAllocatorRef allocator);
extern IOReturn IOAVServiceCopyEDID(IOAVServiceRef service, CFDataRef* x2);
extern IOReturn IOAVServiceReadI2C(IOAVServiceRef service, uint32_t chipAddress, uint32_t offset, void* outputBuffer,
                                   uint32_t outputBufferSize);
extern IOReturn IOAVServiceWriteI2C(IOAVServiceRef service, uint32_t chipAddress, uint32_t dataAddress, void* inputBuffer,
                                    uint32_t inputBufferSize);

#define BRIGHTNESS 0x10
#define CONTRAST 0x12
#define AUDIO_SPEAKER_VOLUME 0x62
#define AUDIO_MUTE 0x8D
#define SLEEPTIME 5000

struct DDCWriteCommand {
	UInt8 control_id;
	UInt8 new_value;
};

int main(int argc, char** argv) {
	IOAVServiceRef avService = IOAVServiceCreate(kCFAllocatorDefault);
	if (!avService) {
		NSLog(@"This is a tool to change the brightness of a single external display connected to an M1 Mac via DDC/CI. Do you have an external monitor? Are you on M1? Are you root?");
		return 1;
	}

	IOReturn err;
	UInt8 data[256];
	memset(data, 0, sizeof(data));

	data[0] = 0x82;
	data[1] = 0x01;
	data[2] = BRIGHTNESS;
	data[3] = 0x6e ^ data[0] ^ data[1] ^ data[2] ^ data[3];

	char i2cBytes[12];
	memset(i2cBytes, 0, sizeof(i2cBytes));

	NSData *sendingData = [NSData dataWithBytes:(const void *)data length:(NSUInteger)4];

	usleep(SLEEPTIME);
	err = IOAVServiceWriteI2C(avService, 0x37, 0x51, data, 4);
	if (err) {
		NSLog(@"i2c error: %s", mach_error_string(err));
		return 1;
	}

	usleep(SLEEPTIME);
	err = IOAVServiceReadI2C(avService, 0x37, 0x51, i2cBytes, 12);

	NSRange maxValueRange = {7, 1};
	NSRange currentValueRange = {9, 1};

	if (err) {
		NSLog(@"i2c error: %s", mach_error_string(err));
	} else {
		NSData *readData = [NSData dataWithBytes:(const void *)i2cBytes length:(NSUInteger)11];

		char maxValue, currentValue;
		[[readData subdataWithRange:maxValueRange] getBytes:&maxValue length:sizeof(1)];
		[[readData subdataWithRange:currentValueRange] getBytes:&currentValue length:sizeof(1)];

		NSLog(@"Current brightness is %i", currentValue);

		if (argc == 2) {  

			int nextValue;
			nextValue = currentValue+atoi(argv[1]);

			if (nextValue<0) nextValue=0;
			if (nextValue>maxValue) nextValue=maxValue;

			struct DDCWriteCommand command; 
			command.control_id = BRIGHTNESS;

			NSLog(@"Setting brightness to %i", nextValue);

			command.new_value = nextValue;

			data[0] = 0x84;
			data[1] = 0x03;
			data[2] = command.control_id;
			data[3] = (command.new_value) >> 8;
			data[4] = command.new_value & 255;
			data[5] = 0x6E ^ 0x51 ^ data[0] ^ data[1] ^ data[2] ^ data[3] ^ data[4];

			for (int i = 0; i < 3; ++i)
			{
				err = IOAVServiceWriteI2C(avService, 0x37, 0x51, data, 6);
				if (err) {
					NSLog(@"i2c error: %s", mach_error_string(err));
				}
				usleep(SLEEPTIME);
			}

			NSData *nsdata = [NSData dataWithBytes:(const void *)data length:(NSUInteger)6];
		} else {
			NSLog(@"Please specify a value between -100 and 100 as an argument to increase/decrease brightness by the desired level.");
		}
	}
	return 0;
}

Here is a much more improved version to control the display:

https://github.com/waydabber/m1ddc