I used to have trouble waking up too. What worked for me was having a strict morning routine that I followed the moment I woke up. I found that sleeping 10 or 12 hours wasn't uninterrupted; I'd often wake up briefly in the morning, realize I was very tired, and quickly fall back asleep. Once I had a morning routine, it became my first priority when I woke up to fulfill that routine and would allow me to pull myself out of bed earlier. Typically my routine is to get ready for work Monday through Friday, but this habit of waking up when I've finished sleeping without an alarm also persists through the weekend.

If I stay up later than usual then I'll often wake up at the usual time and just sleep a little less, but sometimes I'll sleep a bit longer. Either way I still feel very tired breaking out of my schedule. I'm also lucky enough to have an employer allows a relatively flexible schedule so I could experiment with this for a while.

A hard cut of caffeine after 10:30 AM, cutting blue light (and screens if possible) after sunset, and if nothing else having a kid can do it.

Or, try camping for a few days. Try not to look at your phone after sunset.

I grew out of it without doing anything in particular

How much is increasing exercise ?