> we've all collectively heard nearly everything that you can suggest
Have you heard it suggested that insomnia is frequently a symptom of ADHD?
I have ADHD. It was undiagnosed until I was 22. And, through my whole life until that point, my sleep was "delicate" in the way you're describing, requiring careful set-up to avoid insomnia.
Then I was diagnosed with, and medicated for, ADHD. Sleep is pretty easy now. No care required.
One reason for that might be the way the medication (which I take in the morning) affects me as it's wearing off in the evening—it creates a feeling which is sort of the opposite of insomnia. With insomnia, there's a grating sensation that comes from the fact that you're still awake and a weariness demanding you sleep, but your brain won't shut up, so you can't. With the stimulant dose end-phase, meanwhile, there's no sensation of tiredness at all, but your brain is very quiet, not spontaneously thinking anything (in fact it's rather hard to spontaneously think), so you can fall asleep, even though there's no compulsion to. You just decide to, and then... you do.
Another reason might be that ADHD is apparently the brain being stuck in a state below the average behavioral-arousal set-point—essentially, being always half-asleep. As such, you don't get tired when you "should", because your brain wasn't working as hard. A car that stays in neutral half the time will still have a half-full gas tank at the end of the day. (This is also why people advise exercise for insomnia, but physical exercise doesn't really cause a brain with ADHD to engage any more than usual.) Whereas, a brain with ADHD that is fed a stimulant, will have its behavioral arousal level modified toward the normal level, expending more metabolic energy throughout the day, generating more waste products, and thereby reaching a point where it demands to enter a maintenance phase (sleep) sooner.
No matter the reason, I have to say that it works.
Interestingly, according to my psychiatrist, even if you don't think you have ADHD, other neurologically-founded sleep disorders (e.g. narcolepsy) are mostly thought to be disorders of behavioural activation as well—and, as such, are treated with stimulants, just like ADHD. They may as well all be different presentations of the same underlying pathology, given that the treatments for them are interchangeable.