Could she wait until the US summer hols and then move over? Have you asked how she would feel about that?

Otherwise, I'd suggest shopping for a good school in the area where you are hoping to move to. Then if she starts at that school (even if she has to bus/tram for a few months) then it's an impetus to get the house finished and the moving plans underway! kwim?

She's 15 and willing to uproot enough to change countries, the school thing is just another little bit of instability in amongst other big instabilities. Of course it's offset by being a (hopefully) more harmonious home environment, and all the benefits that flow from that. To put it in perspective I left home and earnt my own living at age 16 then put myself through uni, I'm sure she can handle a bit of to and fro-ing on the bus or a school change!

You could even present her with some options and see which she prefers, eg option a) move out at Christmas and change schools mid year, option b) move out here mid year and start new school then, or option c) move out at Christmas and commute to school until you have moved. You might be surprised at what she picks when faced with that choice.

I don't think you should just decide to not move for the sake of DSD. It's too important for your wellbeing. This is one of those times where you need to put yourself up near the top of the priority list, kwim?