I would show him the door the next time he threatened to leave...
But that's just me.
I don't respond well to emotional blackmail. Especially not from full grown adults.
What are you even getting out of this relationship??????
No need to answer a stranger on the internet, but I would seriously consider making a list.
Just to see for yourself if its worth it.
Relationships operate on a currency too, and if you're not getting your (emotional and mental) money's worth, then my friend you are being robbed.
From what you've written you're being robbed blind, financially too at that.
Seriously, make a pros and cons list and see why you actually keep him around and also check if its worth it to invest another 8 years with little to no return.

I'm really sorry for what you've been putting up with, I admire your tenacity though, I would not have lasted a year under those circumstances.
We shouldn't keep making a mistake simply because we've already started.... Life's too short