It's difficult, OP.
I can't help feeling that you want to have your cake and eat it.
First of all, your dad. One assisted living facility is much like another, in terms of where it is located. What really makes a difference is the quality of any care they need, and regularity of visits from friends and family. Your dad might have a couple of years left, or he might have 10 or 15 years left. Either way, the likelihood is that he will reach the end of his life at a time when his great grandchildren are still at school. When someone is dying, you need to visit them now or not at all. That means that your son, DIL and grandchildren will need to get in the car and make a four hour trip to see him, regardless of what is going on with them at school. And it means that for however long he's got, he just won't get regular visits from his grandson and great grandchildren, and he won't get to see them grow up. Even if they visit several times a year, that won't be a lot compared to living close by.
Then there's you and your husband. You're moving a long way from home which means you'll need to create a whole new social life for yourselves, as well as making sure that your dad is properly taken care of and not lonely in his own environment. Maybe you're up for that now and confident you can make it work. Maybe you can. Or maybe it'll be more difficult and lonely than you anticipate, or maybe one of you will receive a life changing medical diagnosis in a year's time. Either way, your son just won't be able to be present for you in the way that he is now.
This decision doesn't seem very future proof if you ask me. What happens if you or your husband ends up needing more support in the short to medium term than you anticipate? Are you expecting your son to drive to see you every two weeks? Every month? Or will the three of you uproot and move back to where you came from to be closer to family?
You have the right to live your own life and make whatever decisions you choose. But this is not a decision I personally would make at your stage of life.
I have to travel for about 6 hours to see my parents, and cross an international border. Never thought that part would be a problem, but then Covid happened and the borders were closed on and off for the best part of a year, when I was having my first baby. Thanks to Brexit, if one or both of my parents needed to go into a home, I could not bring them to live near me, because they now need a visa to live where I do, and they wouldn't get one. So I would have to either leave the bulk of the responsibility to my brother and just do what I could, or uproot my entire family and move back to the UK. And again, thanks to Brexit, my husband would now need a visa to live in the UK. My parents are a similar age to you, and my dad was diagnosed with Parkinson's disease not long before I moved. I felt terrible about leaving the UK, but it was that or break up with my long term partner, who was not realistically able to leave the UK. We travel to the UK to see my parents about three times a year, and they currently travel to see us a couple of times a year. When their health no longer allows them to do that, we will have to travel to see them more often. It is hugely time consuming and expensive. But I was the one who made the decision to move, and so it is just part of the price I will have to pay for my own decision.
If it were the other way around, and my parents had moved far away at their stage of life so that they could pursue hobbies, I would be really upset. Especially at the implication that the hobbies were more important than close and regular contact with their grandchildren.