There are lots of ways of looking at this dilemma but it’s quite interesting @littlealex2021 that you’re only listing your Pros and your wife’s Cons.
What are the lists the other way round?
My personal position is that we live in the countryside and neither of us would want it any different, we love all the positive benefits and don’t find the negatives that other people talk about eg have never sought to acquire a pint of milk at 11pm either when we lived in town, or since. You do need to plan and shop in an organised way though and that’s not everyone’s style.
For me, to counter the benefits of lifestyle, space and easing financial pressure, moving further away from an elderly parent at this stage would be a hard no. That’s without considering your children’s situations.
Secondly, check your figures, the cost of relocation is high and at 53, moving out of London with two teens, there’s a chance that your choice of home now wouldn’t be the same as you’d pick as ideal in say 5-7 years time.
We’re an hour’s train from central London. That’s £50 for a day return, no tube included. Fares rise every year. If you have to park at the station that’s around £6 and the car’s out of use all day, otherwise the person wfh has to do four journeys there and back. You need to factor in hidden costs like this and say the price of fuel, rural broadband, groceries etc depending on just where you’d go. Countryside living is often more expensive than city.
In your shoes I’d wait. Let your youngest finish school, see where things are by then with MIL and then think again. You can spend a good chunk of time really researching where you’d like to live, to suit all of you, particularly if you envisage your DCs remaining in the family home for 10-15 years.
60-90 minutes out of London covers a vast and varied area, you need a lot more knowledge.