I work with families who move countries. Before they moved I made some suggestions to them, eg about how to support language (remember these kids spoke good French). They thought I was exaggerating the difficulties and ignored all the advice. 'Kids are resilient' 'kids learn the language easily'
To their son's detriment.
Oh gosh, this. I am a permanent resident in a non-English-speaking country, and it drives me mad when parents dump non-local-language-speaking adolescents in the local schools in this way ("Kids are like sponges!" "They're resilient, they'll cope!"). It almost never works out and causes a lot of stress and unhappiness, and sometimes long-term resentment; it also burdens the school system and takes teacher time away from other kids who need help.
Even switching at younger ages (10 or under) needs to be considered carefully - parents need to be prepared to support their child thoroughly in the new language, AND keep up English at home. It can be worth it, but it's extra work and not for every family. And 10/11-ish is pretty much the latest age for making it work, IMO.
After that age, better to stick with English school while putting them in French classes and French-language extra curricular, so that they can forge their own path to bilingualism in their own way, without putting their general education at risk and suffering enormous stress at the same time.
I think I will definitely stay here with dc for year 6 & dh can move.
I think that might be ideal for a year or two. Your DC can then be involved in decisions about boarding, IS, and other options.