Thermoregulatory-related chronic insomnia, ambient-temp woes

For months now I've had particularly debilitating chronic insomnia. Either onset will take hours, or sleep following my half-way point will remain light and dissatisfying. Bad nights will yield me about 3-4 hours, good nights up to 6.

I have a convectional heater in the bedroom at my control with which I will under/over compensate for outdoor ambient temperature. Fiddling with it frequently has been my approach, as, layering sheets and blankets often results in frustration at onset; I get too hot under them, too cold without, in spite of the fact that this was once upon a time the way I would sleep. As such I keep a single bedsheet (with extra at my feet) as the only constant, and fiddle with heater to achieve thermoneutrality.

When sleeping in the nude, thermoneutrality is thought to be around 28-30 degrees Celsius ambient, while with bedding it's around 16-20, depending on the individual. I've had mild success some nights just setting my temperature somewhere in between (24,24.5) and have attempted to recreate those conditions. Fluctuating temperature outdoors renders this near futile some nights.

Let me rule this out: I practice what I believe is adequate sleep hygiene. I go to bed and wake up at a regular interval, exercise early, I stay away from electronics for a spell and do some yoga, I read for 30-40 minutes, I take a hot shower an hour ahead of sleep, it is pitch dark in my room, etc.

If thermoregulatory issues are familiar to you, what conditions do you hazard are the most consistent/reliable way to manage temperature regulation? Particularly when dealing with deviations in ambient temperature.

My hunch is that I need a more reliable bedding "stack" to test at maybe 20 degrees (18 seemed uncomfortable), such that I would rely on bedding to adjust comfort rather than relying on getting ambient temp just so. What I've read tends to point in this direction, but my attempts have been failures.