DAC1 will give you an analog voltage and not a PWM signal, if this is your intention with
pinMode(DAC1, OUTPUT); analogWrite(DAC1, 400); // just setting something up for the pulses
And how have you wired your setup (e.g. which pin triggers D3)?
OK, I’ve now tried out your code from above (D1 -> D3 -> scope Ch1, D0 Ch2) and I get a good copy of D1 on D0 with a delay of 1µs
Have you tested with my code?