Your body has a clock.
Learn yours. (sleep earlier equivalent to your time).
I've trained myself to wake up at 7:30 AM on the dot for the past 5 years everyday without an alarm.
Hack yourself.
I'm similar. I wake up between 3 and 4 then fall asleep again at around 6 to wake up around 07:30. To remind me to start working I have built a Sonoff Basic into my bedside pedestal fan which turns the fan off at exactly 08:30 because my contract stipulates that I need to complete a COVID cheecklist by 09:00, else a leave day will automatically be deducted.
The turning on and off of the fan is quiet enough not to disturb the wife, so if I have something critical I have to finish early before the bosses hound me, I use the switching off of the fan as an alarm. Once I don't hear it's sound anymore and I don't feel any wind, I know it's the Basic that triggered the action.
There's the little problem with loadshedding now that will stop the fan sometimes at 4 or 6 thereby waking me, but I'm looking at getting a rechargeable fan to overcome that issue.