Currently in BC and shall answer what I can....
1 - Earning $175k between you both will be plenty to live comfortably, depending on where you want to live. We live a 40 minute drive from downtown Vancouver and we survive on around $90k. However, we don't have expensive hobbies like skiing etc and we only run one car with no car payments etc.
Food, car insurance and phone contracts are all stand out more expensive than the UK.
2 - School system is different. Children start the September of the year they turn 5, so all kids who were born in 2017 started this September just gone. They also don't have as high expectations of the young ones and from our experience it starts to level off by Grade 5/6. We came having had our first child complete R-year 3 in the UK and our second has just started Grade 3 and he is not at the level our first born was at at the same point. However, having seen our eldest now reach Grade 6 I don't have any major concerns.
Summer breaks are 10 weeks but you adjust and I can't imagine only having 5/6 weeks now. It makes the autumn term long as the kids go straight from September through to December with only a few long weekends for breaks.
3 - We have integrated really well. Again, this may depend on where you settle. There are areas around where we are that are predominantly of certain backgrounds, e.g. South Asian etc and these would be harder to settle in as some neighbourhoods can be very culturally driven. This isn't exclusive so just do your research before deciding where to live.
West coast is not really french speaking. There is french on the signs and on labels but no one I have met really speaks french. They learnt it at school like some of us British kids do but nothing more than that.
4 - I can't answer this as we came in on a Religious Worker Work Permit which then allowed me as the spouse to have an open Work Permit. We didn't have to have any savings, we just needed a job to come to, which we did!
5 - We enjoy our life here. We like the open spaces and the access to beautiful outdoor experiences. There are always pros and cons to all countries so don't move to run away from the UK. Do we have a better quality of life? I don't know whether we do but we love it out here.
You also need to be aware that jobs offer far less annual leave - we get three weeks and that is generous. However, there is a long weekend nearly every month and people take full use of their weekends. Work days also generally finish a little earlier, it's more a 8-4 day with some variation.
We like the more defined seasons, i.e. hotter, more consistent summers and snow in winter (although the snow isn't consistent where we are, we just happened to have a good couple of years!)
Feel free to ask anything else.