Hi Nat - I've been on Mirtazapine now for 3.5 years, at doses of anywhere between 2mg and 30mg.
Everyone reacts differently to different ADs so my experience could be completely different from yours, but for me it is the best sedating treatment there is, of any kind. It worked for me in the depths of insomnia hell when nothing else did, so if sleep is your main problem i would highly recommend it. I never had depression or anxiety (except as a result of prolonged insomnia) so can't fully comment on how good it is for those but i hear only good things. As you say I did gain approx 2 stone on it - because of the way it affects your metabolism it is very hard not to gain weight on it - but as you have lost 2.5 stone this isn't an issue for you!
In terms of dose - I have never personally found there to be truth in lower doses being more sedating, though I realise this is discussed a lot. For me it was more sedating at the higher doses (so much so that I would struggle to stay awake past 8pm even before taking the dose). My psychiatrist also didn't hold any weight to the theory of lower doses being more sedating.
The start up side effects can be sticky (like any AD) but they do subside after a few weeks and then I had / have none at all. I always think it's good to start off on a lower dose (eg. 7.5mg) and work your way up once your body has adjusted.
One last thing I would say is that in my personal experience no AD is going to rid you from the symptoms of menopause,. They may reduce some but for me the side effects i experienced such as palpitations, night sweats and insomnia were all present regardless of what AD and at what dose I was on. I'm only saying that because if your issues ARE hormonal I think a "two pronged" approach would be best - ADs can be great tools and definitely great to get you out of this initial hole, but perhaps in parallel to trying to balance your hormones? However sometimes it can be hard to know what's menopause related and what isn't, so you never know - it could help you on its own.
Feel free to ask any more questions, there's not much I don't know about Mirt! (Although as I am careful to say, everyone is different

)