I am a twin who didn't accelerate because my brother didn't need the skip. I totally get it. I got it then when I was a kid and I definitely get it now as an adult.
The one thing that I would have really liked, though, was more opportunity to be involved in the groups and activities that my older friends were able to be involved in. Kids two years older than me were my true peers and it was so frustrating being separated from them just because I was younger. I also found the same situation with academics that I found with social situations. I wanted to learn things and would have loved being able to be in classes with the older kids. So, I think your idea of trying to find a school that includes both middle and high school is a good start.
I know it's a tricky road, so I commend you for being so thoughtful about it. Just thought I'd share my perspective on it in case it might be helpful. Not sure if it applies, though, so YMMV
