Hi OP. This sounds a hard stage for you. You've put in a lot of effort and home is not a happy place for you right now.
Just for insight, because you ask, I have 4 DC and 3 DSC and they are all teens/early 20s. I have lots of experience and so I might go on a bit - but I hope I can help you explore a workable solution. It sounds like your instinct is to run for the hills! I can understand that.
I am not going to underestimate how alienated you have felt being the adult partner in the home, and yet not having any control. Step mums get this and it is hard for us, and most say the hardest thing we have done.
As a word of encouragement can I suggest to you that you have done the hardest bit already and the only way now, is up.
Your DP's 4 are late teens and early 20s and they WILL move on in a few years.
You might well be experiencing empty nest having lost your boy to uni, and the 7 year itch, on top of not being on the inside of the family unit.
Can I suggest you talk to DP (who cannot call the shots as to whether you stay or go). Talk again I mean, because obviously you have talked.
He has said he would split from you if you leave him, to live elsewhere. That is his perogative, but sounds like a threat. So be careful what you wish for.
You have detached a little from your son because he is away at uni. He has his own life. Similarly, your step children are older and have their own lives, and you can detach a little too.
The way forward is to Keep Calm and start to be more self- preservationist !! Draw the line in the sand and set some house rules. You do have a right in your own home to stand up for yourself. You are not running a youth hostel!
Your Dp can cope with a little change for the better. The kids can learn new regimes. Over the last 7 years many things will have changed. Nothing stays the same. You are in a limbo where things are on the brink. The only positive thing about being in limbo is that it doesn't last long by definition, and that you have an opportunity for a change of direction.
Think about staying in the relationship and in the home. Losing relationships is hard and sad. Building richness together with a partner is supportive and heart warming.
Acknowledge that you DSC are part of your life but make DP understand you are not a doormat. It's in everyone's interest that you assert your right to be heard in the home.
You are reassessing in your mind. Try to find workable solutions to what you will and won't tolerate. Move your homelife in a more positive and sustainable way of living. Box clever and don't do the teens down- most teens are self centred and you would be somewhat irrelevant to them compared to their bf/gf.
(If you want an example of one of our house rules (relatively new rule after one DSC returned from uni) - then for example, I said "NO laying on the sofa to watch TV and being sleepy. Go to your room if you're tired". I felt it was an intrusion on the shared living room. The young adult pulled faces, argued a little and I stood my ground. At first my D P sided with the thinking that it's ok to lay on sofas, why couldn't his DC be allowed to just do as they wish? I said to DP, that we are a big household and we can't all do it - there isn't room, and I won't have it. If he felt it was okay then I look forward to turning up at the kids first place and laying on their sofa, scratching, sweating and snoozing and having an opera on tv, that no one else enjoys hearing! Hubby grumbled, but I find that now the kids don't expect to be allowed to slob out selfishly in the living room - even though it is okay if we are all flopped and watching tv. So complex rules can be enforced. But you have to stand up for yourself.).
Understand yourself too - perhaps that you are at a loss without your own boy and you are outnumbered by teens and THIS IS HARD. But it can get easier and the good times are to come!
I hope you can see some positives amongst all the hardship OP.