I could have written yr post Op but, from yr husbands perspective.
I mean this with kindness but i eventually left my now ExDH and divorced. I was fed up of putting mine ( and our children's) needs secondary to his. I wanted my own freedoms to make my own decisions about where i live,my career, as did my DC. We were miserable for yrs, where he moved us to. It suited him, not is.
With respect he (yr husband) does not have to make friends anywhere, if he does not want to or fails to. There is no law saying that. If you want him to make friends you cannot force that. He is travelling Sun-Fri, that is totally unreasonable. I am not surprised he is stressed and depressed.
With kindness, this is why i will never marry or be in a long term relationship again. I will never ever again move, to accommodate the wishes, desires/career progression of another. I stay single so i please myself and make my own choices ( with my DC in mind always) about where i live and work. This kind of example you have written about, shows exactly why long partnerships can negatively affect one in that partnership, if they are tied and decision making is joint. Inevitably someone will come off worse somewhere along the line.
You say his bosses are being unreasonable but do you mean they are not happy he has now moved miles away from his role/office? Do you mean they are not accommodating his needs/requests , so that they fit in with yours?
"And pressure from him that this is indirectly all my fault.
To cap it all there's more support here but it's still negligible and he has few friends in either place, I suspect a part of this is due to his inability to forgive or make new friends. There's always a reason why he doesn't like people. Some valid, some not".
It kind of is indirectly your fault.
He is allowed to feel as he does. Negligible support is irrelevant. If he has had issues with friends in the past and does not want new ones he may be using and implementing boundaries. That is his right. You seem to be mistaking this for mental illness.
He has moved miles away from all he knew and is now away from his children, during the week , staying with his parents, all to support your career.
He is not ill, he is fed up and having to compromise what he wants in his life to support your career. He is most likely staying at his parents as, a studio or 1 bed flat 5 nights a week on yr own can be lonely.
He is unhappy and frustrated. How would you feel being away from yr children 5 nights a week?
Why are you not the one staying away? How was this decided? Why don't you move away 5 nights?
"There's possibly depression present and he hasn't bonded with our youngest".
Lots of women suffer from depression and do not bond with their children as do Dads. This is not uncommon. What help have you put in place/asked for to help him? How have you addressed this and helped yr husband?
"if it's always overwhelmingly negative then he can bugger off somewhere else".
Yes, this may well be what is best for him. Allow him the freedom to live within a reasonable distance of his work and see/have his children 50% of the time.
With kindness OP, this post smacks of, he is not doing/behaving how, what suits me so he is some how faulty . If you want to move to peruse yr career then fine. It is unrealistic to expect another grown adult and shared children to share your vision and move with you. It sounds to me like you are on a solitary path and perhaps marriage/long term relationship does not fit with your personal life goals.
He is not ill, he is fed up. There is a massive difference. Persue yr career by all means but do not expect yr husband and children to make such large life sacrifices to enable that. You may well be suited better on your own Op.
Personally i found yr suggestion for yr husband to " bugger off" hugely disrespectful and dismissive. Maybe you leave and travel 5 nights a week?