@Fallingrain - I have no advice on whether you should try to encourage your DH to move or not, but I do have experience of relocating my family (including our then 3 & 5 year old DDs) 200 miles to be closer to my DM and her 'D'H.
We were fed up with the 4 hour each-way drive to visit them, and DM was keen to see more of (and help with) our DDs, so we made the move. We had 5 wonderful years, where my DM grew incredibly close to our DDs, and became my best friend. Unfortunately, her 'D'H decided to take a new job, 100 miles (a 2+ hour drive each-way) from where we were all living. DM, the DDs and I were devastated when they left.
6 months after they had moved away, my DH and I were talking how much we were missing my Mum and about our future (our DDs were then 8 & 10); DD1 was in year 5, so would be starting Senior School in 18 months. Where we were living was beautiful, but very rural and there was only one local Senior School. It became apparent we were going to have to move because:
• The only local Senior School was tiny and after doing some research, we decided it wouldn't be the right place for our girls
• We wanted more opportunities for our DDs
• Our driveway came out directly onto a major A road, with no pavement, so to go anywhere, we had to go in a car, so no chance for the children to go out independently until they could drive
• It wasn't safe to walk at the edge of the road to the bus stop about 700m away
• very little to do there, unless you were into a couple of very specific sports - our DDs weren't.
So, we decided to move and chose a city an hour from where my DM was living. We wanted to move quickly, as we wanted our eldest DD to start and finish Year 6 with some of the same children she would be going to the same Senior School with.
Initially, our DD's reacted with a mixture of excitement and sadness. We had a few tears about leaving their friends, but it only lasted about 2 days. What we did to try and help:
• We consistently talked about the exciting benefits of living in this new city
• Talked the good sides up; new friends, being closer to Nanny, more opportunities (drama clubs, a girls’ cricket team, computer coding lessons, rock climbing…), loads of clothes shops, FIVE swimming pools within 5 minutes’ drive of the new house (this was a BIG selling point!), nearby woods to explore, a river with a cycle path and finally having their own bedrooms and no longer having to share!
• Arranged a Leaving Party-Sleepover (thank goodness it was summer, and we could stick all 10 of them in a massive tent in the garden!)
• Worked hard on making arrangements for them to see their closest friends over the summer and I have made the effort to stay in touch with their Mums, so we've met up 6 times since we moved
• FaceTime calls around once a month with each of my DDs 'BFFs'
• Facilitate these get-togethers by offering to drive either the 45 miles to collect or to return their friends home
We've been here 2 years now and it was absolutely the right decision for our family. Their 1st day of their new school went like a dream - although we did have a few tears early in the 2nd week as their 'new girl novelty popularity' started to wane. But by the end of the 3rd week, both were settled, happy, had made some lovely new friends and they were starting to be invited to birthday parties by October half term.
When DD1 moved up to Senior School, she had 5 children from her class at Primary in her Tutor Group, so the first few weeks were less daunting than if she had had to walk in on her own. Having girls she knew with her gave her the courage to approach others, so she was able to make friends more easily than she might have if she'd also been alone and her confidence has really grown.
Personally, I enjoyed being closer to my DM, but it wasn't close enough that we could just pop round after school or work for tea, or that I could help out if she was poorly (particularly if we travelled during rush hour, when the hour journey could often become two), so we still didn't see each other and as much as we would have liked – which is why I was delighted when she left her husband last summer and moved 10 minutes’ walk from my front door! 
@Fallingrain I think you have the right idea in making the move before your eldest starts Secondary. As I've said above, it made a real difference to DD1 and although it had its tricky moments, if you ask our DDs now, they'd choose our new city over wanting to live anywhere else (although it's possible DD1, might choose to live in the Harry Potter Studio Tour Building 
).
DH was the main breadwinner where we used to live, and although he started applying for jobs in our new city once we’d decided to move, he didn’t get an offer he really wanted until we had been here for 9 months – so had to do a one hour each-way commute in the car, along a notoriously awful road for that whole period. So, along with making sure the kids move at the right time for them, I'd suggest (if your DH starts to show any enthusiasm for moving), that you help him to secure a role in the new area as quickly as possible, so that he feels he has a connection to the new place, a personal purpose for his moving there, along with introducing him to your friends and their partners 1-2-1, over dinner so you can talk properly and your DH feels he's really getting to know people. Perhaps you could even ask your friends to help you do a 'Welcome to Northern Town Falling Rain Family!' party (you could organise it, but a bit weird to actually 'throw it' yourself - get one of them to do the invites!) I think otherwise I might feel quite resentful if I moved my whole family away from a place I liked, where I had a decent job I enjoyed and ended up in a place I didn't want to be, with no job, having left all my friends behind.
I live the idea a PP had about testing the waters to see if your parents might be interested in moving (closer) to the town you currently live in – might be a good compromise?
Ooops, apologies - I've been writing this in chunks as I potter about doing jobs and hadn't realised how much I'd written - I hope there's something relevant or useful in my ramblings!
I hope everything works out in the best possible way for all your family OP 