Fideliney
As a pp has said, this level of hoarding does always have it's roots in psychological distress/ MH issues. It can be treated and there are specialists don't around the country who, if you DH was willing to work with them would likely be able to really help him.
Sadly, as in so many compulsive behaviours he may have to actually 'lose everything' or be at the precipice of doing so before he has the motivation to change, so sadly a I think all you can do is make a decision based on what you and your Dc need right now and then follow that course, with or without him.
It sound that as the Dc are at a critical stage in their education upping sticks is not a viable option, so how about doing as pp said, informing DH that as you will soon be returning to work FT you will require a space which will not be available to store anything other than your own work items. hence you will be disposing of any items that are in this room after say, one weeks time, into recycle/ skip/ rubbish. Stand form on this and if you have to carry it our then do place a lock on the door. This is not unkind it is to save your own sanity and enable you to work.
If I were you I would probably stop setting up sheds etc, as it is actually enabling him to continue his behaviour whilst minimising the negative consequences for himself. Continue to be clearance nd firm that other areas will need to be kept clear ( ie communal areas, kitchen, loos, hallways etc and that you will organise clearance by X or Y date.
Again this is to keep your sanity short term until you can make a proper plan to leave, as I suspect this is what it will come to.
Good luck