What is the treatment for delayed sleep phase that is best and has fewest side effects?

Melatonin + light. 1) choose an initial wake up time you can adhere to at the start, 2) upon awakening, get exposure to bright light, 3) later that evening, take melatonin 3mg about 3 hours before your sleep onset time the night before, 4) wake up 1/2 hour earlier the next day and get bright light, 5) repeat 3-4. Eventually, you will correct your sleep-wake schedule.
Sleep hygiene. Improve sleep wake cycle , avoid evening light exposure. One need to improve sleep hygiene slowly and going to bed by 15-30 earlier every 2-4 week slowly and waking up also early 15-30 minutes every 2-4 weeks it takes many months to take effect. Use alarm to wake up including weekends. This will be slow and takes months to a year to improve. Melatonin in evening may help. Discipline is a key.