That's a beautiful area OP. I love Portsmouth. You should also look at living in Maine - there are many lovely areas there too, and you'll be in a more liberal state than NH with (IMO) on average better schools, although what really matters is what the schools are like in your town.
I'm further north in Maine. Have lived here more than 13 years and love it.
I never felt lonely - I have always worked either self-employed or in a job. DH gets 25 days a year holiday, I got 33 in my last job.
I've found people to be friendly and helpful and welcoming.
My kids have had an excellent education in the local school system, and one went on to a magnet school. One child with a 504 Plan - was quickly diagnosed and school has made all the requested adjustments.
The average class size is 15-20, the schools are well-funded and have continued to teach right through Covid. Every student is provided with a laptop or similar device.
My oldest is now at university and receives a scholarship, so it costs about the same as university in the UK would cost.
Always makes me chuckle that people say US schools are no good. Many of the kids we know have gone straight from US high schools into UK universities with no problem.
DD's US school had a Cambridge-educated principal and an Oxford-educated English teacher. They encouraged DD1 to go to Oxford or Cambridge, but she chose to stay in the US.
Nurses earn good money - you should definitely look at what training is required if you plan to stay longer term.
I agree that the cost of living in higher, but it's not crazy in northern New England like it is in places like DC, NY, California, etc.
We've never paid double tax, and DH is American and has filed his taxes every year as an adult.
We arrived with children aged 2-6, over 13 years ago, and we've all been very happy here.
We spend the summers swimming/paddleboarding/kayaking in the local lakes, rivers and beaches. Hike in the mountains. Winter we ski, snowboard, snowshoe, and hike. Yes it's cold, but it doesn't rain anywhere near as much as in the UK. My biggest gripe is how everything gets so icy in March.
You do need to be prepared to deal with a lot of snow. We pay someone to plough our drive after big storm, we own a snowblower, and after a storm we will dig out the pathways to the house, blow paths for the animals in the garden, around the cat/dogflap, in front of the garage. It can be hard work doing all that before you leave for work.
We have an acre of land and a lot of trees, so that takes a fair bit of management.
We put studded snow tyres on the cars, and for a long time we just had 2WD cars, although we now have a 4WD and a 2WD. The roads slow down briefly during snowstorms then get back to normal within a few hours.
The federal politics are currently horrendous and we're all desperately hoping Trump is voted out.
Most of the politics that will make a difference to your life are state level and even town/city level. For example, our Covid response at a state level has been excellent, which has made up for the lack of a federal response.
I could go on forever, but if you do have questions feel free to PM me.