Hi Jules, long-time AD user here (mostly citalopram)! That said, I haven't personally used mirtazepine, but I've tried escitalopram, citalopram, fluoxetine, sertraline.
I've been on and off AD's a lot, largely as a refusal to accept that I'm mentally ill, but I always have to go back on. And finally, more relevant to your post, whenever I start taking anything, I consult lots of forums to get me through those lovely early stages. What I have noticed is that quite a lot of people with insomnia seem to think Mirtazepine is brilliant for sleep, and it does seem quite a well-tolerated one from what I've read. Surely it wouldn't hurt to try it and see how you get on?
As for if you need to come off - it's so variable between individuals and drugs. Sertraline is meant to be horrific to come off of, but I just stopped taking it and was fine. Escitalopram, on the other hand, I had nightmares about being kidnapped and held at gunpoint! So you just never know. If you do need an SSRI and need to come off Mirtazepine, probably you can switch straight to another SSRI without withdrawing before starting a new one. That would just be needless difficulty.
I've been offered propanalol to take alongside citalopram, didn't take them. But this was years ago so maybe the advice has changed?