Your situation sounds similar to how ours was. There is a similar age gap between my ds's. I decided I'd probably tandem feed if needed but at around 4-5 months pg with ds2, ds1 self weaned. Apparently the taste of breastmilk can change when you are pg, also in some cases your milk supply can decreased. I think this happened in my case but this doesn't happen to everyone.
Anyway, one night I offered ds1 cows milk in a cup and he took it quite happily. I was still feeding him in the morning (mainly because it gave me an extra 20 minutes in bed ) but he gradually became less and less interested until one day he flat out refused.
So you may find she self weans soon anyway.
We also moved him into his own room when i was about 6 months pg. Fortunately he settled quite easily and was quite happy in there by the time ds2 came along. If you are doing it around Christmas, could you make it exciting for her by getting some presents specifically for her new bedroom.
As for toilet training, well I bottled it and waited until after ds2 was born. I just couldn't be doing with the any extra stress of dealing with accidents/regression etc and a newborn. We are only just doing it now (ds2 is 8 months) but it seems to be the right time for ds1. We did try earlier but ds1 wasn't ready anyway so for us waiting was the right decision.
However if/when you think your dd is ready, there's no reason not to give it a try. Apparently girls can be ready earlier than boys.
And as for making her feel involved, there will be lots of ways you can do that. Ds used to love fetching me things for ds2 for example.