I'm sorry OP. I remember 2008 when it all went wrong. We had just moved to a house that was a mistake. The move had cost a fortune and we didn't like it (panic buy when we couldn't find anything).
Then 2008 hit and all of a sudden house prices dropped by 10-20% and our relationship which was already on the rocks became worse. Both our jobs became under threat and DH got put down to a 3 day week. It was an awful time. Like you we had borrowed quite a lot but not up to our max. It was a different time of course and they slashed interest rates to practically zero which did cut our mortgage.
In the end we split up and sold the house getting 50K less than we paid for it (paid 405k, got 355k). We also cashed in our endowment to help but of course with the markets the way they were we would have got less for it. We were in our thirties but didn't have kids.
I know your situation is different and interest rates are on the up and you and DH are not splitting up. However it is amazing how fast it can become very scary. It is like a domino effect. Rates start to go up, prices start to go down, people's jobs became less secure or worse redundancy, there are lots of forced sales, prices drop further and so on.
As you are discovering it is a bit of a judgement call as to the best way forward.
I'm in my fifties now so my advice from someone that got themselves into a pickle would be as follows.
If you like your house and you moved to give your kids space and everything is great apart from the money aspect then I would try and stay. Selling costs a fortune and obviously is the wrong decision in the long term.
How safe are your jobs - I know that's a tricky one to answer but if they are quite safe then I would say, stay, remortgage as soon as you can to get the best rate (and yes get your LTV down to under 60% because that would get you a better rate). You should get your house revalued but if prices in your area are going down beware the value might drop below it's current value and the LTV might end up higher.
If you are on a fixed rate have you calculated to see how much it costs to get out of the rate now and pay the exit fee versus how much you would save by getting a lower rate now which of course you can tie in for a longer period.
Not ideal of course but can your kids share a room for a few years and use the spare room for a lodger. Do you have a separate dinning room which could be converted into a bedroom in the short term so you could have 2 lodgers. If you did that now you could start building up a bank of cash to get your LTV under 60%. Or you could just keep it as a safety net in case of job insecurity.
What I will say is lots of people are going to be in the same boat. Remember all the houses that were being bought in 2022 and so anybody that took a 5 year fix will be having to remortgage next year. Good in so far as you are not alone. Bad in so far as if lots of people suddenly can't afford their house and lots come on to the market prices tend to depress.
It sounds like your financial situation will improve in a couple of years when your youngest goes to school so I would just have lodgers for a couple of years. I know, it's crap but if you get to stay in your dream house for the longer term........
I hope it all works out for you.