I have multiple severe sleep disorders including non-24 and insomnia so it is hard to tell how my experiences match most people but hopefully this might help some. My sense is that when I sleep only a few hours then can't get back to sleep for at least three hours it means my body is interpreting the sleep as a siesta rather than night sleep (unless it is after sunrise). Non-delayed release melatonin makes this more likely for me. I don't think delayed release makes it less likely (at least not right away) but might at least not make it more likely and longer term it seems to help make it possible for me to sleep on a closer to fixed schedule. Magnesium is one of the few that helps me stay asleep though I suspect what might help you most is if you could take a nap earlier in the day. While most people these days already do this part I've found that caffeine in the morning makes it much less likely that I'll have this issue (I can't have caffeine in the afternoon or too much theobromine even in the morning multiple days in a row or I will have trouble sleeping; I imagine the helpful effect will be much less in people who are desensitized).

It sounds like you have a delayed circadian rhythm. The Circadian Sleep Disorders Network has some good info:

https://www.circadiansleepdisorders.org/

I don't agree with many of the suggestions in this article. Advancing your sleep schedule to try to shift your sleep earlier is particularly harmful and can trigger non-24. In my case, part of my body seems to be synchronized with light and part rotates around the clock leaving me rarely all that functional. It is a disabling condition. A somewhat delayed circadian rhythm seems to usually be much less disabling but may also cause similar trouble if severely delayed (many people don't have as much trouble with non-24 as I do, though I didn't have as much trouble for the first 15ish years either).

Waking up at the same time every day is another thing that can help stabilize the circadian rhythm (though I also think it is best to avoid alarm clocks most of the time). Going to sleep at the same time is less important; it is better to go to sleep early if you need more sleep than to sleep in.

I think it is best to avoid medication if possible. What I am using now seems to be working the best of anything I've tried but it has only been six months. At night I take 300mcg 6-hour delayed release melatonin, 250mg delayed release magnesium (though I think non-delayed may work just as well), 2.5mg baclofen (the small amount keeps it working with daily use and using it with magnesium works better than either alone), and 8-9mg diphenhydramine (not enough to help me get to sleep but just for the circadian effect, either 1/3 of a 25mg pill or 3.5mL of 12.5mg/5mL liquid). In the morning I usually drink some tea and take .5 tsp (2.5mL) D-ribose which seems to help quite a bit (I had previously tried it without noticing a positive effect but I think I may have only used it later in the day). In the US there seem to be many companies that package D-ribose but only one company that makes it.