Don't know about CASA, but for the UK CAA, Mirtazapine is disqualifying, even if its intended use is as a sleep aid and not anti-depressant. Your story pretty much mirrors mine. Awful insomnia and nothing apart from Mirtazapine had any effect. 3-4 hours sleep per night and 5-6 wakings during that time. Prior to Covid I had no sleep issues, despite the rigours of a short haul airline roster. I took 7.5 mg for six months and came off it at Christmas. The medics were adamant that I took it constantly for that six months and not intermittently. Sleep has been normal ever since.
My AME does not see any reason why I shouldn't regain my Class 1 after the medics finally sign me off next month. I had some respiratory and blood issues as well but these have now resolved. For me regular bedtimes, no alcohol and swimming really helped and apart from not flying I lead a normal life now, after a pretty grim 2.5 years. With perseverance it is possible. Having taken it, I can see why the CAA do not allow it. Yes I slept really well but was very groggy until 10 in the morning. Coming off it has been like waking up.