I also take a low dose of mirtazapine for chronic insomnia, 7.5mg.
For me there is no hormonal component as I have had it for years and am not deficient in ovarian hormones.
You are correct that at higher doses the sedative effect is lost.
Regarding hormone therapy, it's your body, your health and your money, nobody else gets to say whether you're "allowed" hormone therapy.
This paternalistic attitude among clinicians is now thankfully dying out, and I would say if one provider cannot meet your needs, go somewhere else, don't just give up!