It's ok to leave if your marriage isn't working out how you want it to. Accepting reality can mean many things, realising that things will not get better and that this situation is not for you is one option. 

As to your four points. Point 1 and 4 are things that your husband should be working on. He should make sure there is space so that you can be prioritized sometimes. He should also be working so that he is setting his kid up to be able to be independent. I don't know your ss's situation but it may be a few years after 18 if he is very limited by his disabilites, but that doesn't mean a plan shouldn't be in place. 

Point 2 - fit a lock on the door that locks automatically when the door is closed. You will have to make sure that you have a key on you any time you leave the house but at least the door will always be locked. 

Point 3 - You can have nice things, you will just have to take your ss's clumsiness/vandalisim (you didn't specify)  into consideration. So you will have to keep things out of his reach or keep him out of their way and you might want some good insurance coverage.

Most tween and teens are selfish A-holes. 

Boys are awful in confined spaces.  My son would have driven me bat crap crazy if we were in isolation... due to him being a bull in a china closet.  He could look at something when he was that age and it would break.  And he was loud at that age.  I mean loud.   haha. 

Get a door knob that stays locked if you walk outside.  You can turn handle from the inside but not the outside.   

Most of your complaints are normal IMHO for a boy that age.  I am sure it is magnified ten fold if he is autistic.  And twenty times being a skid.  lol.  But the one thing that sticks out to me is that you think he may not launch.  I could not deal with that.  I would have DH looking into types of school to teach SS how to live in the real world.   Or rethink living with your DH in the same house.  

Day 46 of lockdown.... today has been particularly brutal! Kids now in bed (phew). Mother of a very bored autistic girl here (due to lockdown). I fully sympathise, they have their own unique set of challenges. However it can also be very rewarding. 

My daughter is my daughter, she happens to have autism but that doesn’t fully define her as a human being.

parents just ‘accept’ their kids whoever they happen to be pretty much. I try and find absolute joy in small achievements which tends to makes other stuff easier. My ASD daughter isn’t as much of a challenge as my 21 year old son who got stuck here during lockdown..... ughhh!

I have a friend whose son is high functioning Autistic and she even has been very candid about how hard it is, even being the child's mother. She has said there are days when he is with his BF that she doesn't want to go pick him back up.

Now add the stress of being a step parent on top of that and the reality is you are in a very tough situation. The difference is you have the ability to walk away if this is just to much for you.

I used to hide my door keys from my kids when they were little and I have got a keysafe with a number code on it (works for now till she can outsmart me). They would always be in reach distance from me in case of an emergency though. 

I have an insurance policy with an understanding company (though I am not sure they will always be understanding...)

And every now and again worrying where she will live as an adult keeps me awake at night....

I just re read your post. 

Is he breaking items because he can or is he deconstructing them in an engineering  like’ like way ‘trying to figure out how it works (which is what my daughter does). 

When she goes into the garden she will check the nuts and bolts on the fence then play with toys, then pick flowers. 

Sometimes I’m in her world and sometimes she in mine (if that makes any sense to you at all. 

I would get a lockable cupboard and put away anything you are very fond of, and try some mechanix construction/deconstruction sets. Also, these days people are never too old for Lego. 

If there are signs of high level anxiety eg aggression, stimming, intentionally breaking stuff out of anger, in the U.K. they would consider possibly putting them on anti anxiety medication. This may also have a knock on effect on schooling and homework. 

Autistic people can have back to front melatonin levels (day and night reversed) which also may affect sleep and concentration levels. 

May daughter is currently in melatonin tablets so she sleeps better at night and therefore is less tired and grumpy during the day. 

She would wake hourly or two hourly, then get up at  4am. Everybody was suffering from lack of sleep. 

She now sleeps through to approx 5.30am which is a huge treat for me lol!

 If he suffers from light sensitivity (some modern lighting, new TVs, computer screens and phones can all emit more types if light than they used to. I have to have regular ‘light detoxes’ throughout the day help her concentration. Eg The tv gets switched off at regular intervals throughout the day. I also have to go round switching lights off during the day because she thinks she needs them on even when the sun is shining through the living room window

Somethings are bigger than we are. (((HUGS))))

No advise. Just a ton of compassion.

I live with a similar situation. SS here is 20 and we are really struggling with getting him out of our home. His father and I have reached breaking point. Sadly, unlike the US, we cannot throw him out here, it is illegal to make someone homeless here and he could take us to court if we did. 

I hate to confirm your fears but no, he will not launch, they don't. I have found SS to be super dependent and entitled.

I don't know what solace I can offer you. Person ally I am on the edge of a breakdown living with his behaviour and living with someone I have come to hate. No I will not leave. I want to be with my DH. I certainly never signed up for this tge way it turned out. The BM used to have eowe, and we at least got a break. Then she got a new bf and literally threw SS away (probably because of the trouble he cause with her bf's kids) and even went to the judge to have a paper made to say he shouldn't go to her any more. SS since then stopped speaking to her and so we get his BS 24/7.

What I really want to say is that  a lot of the problem with hi func auto is that people capitulate to them, parents are often just allowing the behaviours because they get worn down by the 'melt downs', society is saying 'he can't help it he's autistic' and that we should pander to their wants. I am only talking about kids on the high function end, those more severely affected are something I cannot comment on.

Step parent comes along (applies to all step kids not just asd) calls BS on some of the bad parenting and the bio makes changes but... The kids have been brought up in a different way for their most formative years, they rebel, ignore etc and the BM certain y will support them. You are on a hiding to nothing. The situation will never change. You have been given some very good advice here. 

SS here is a terrible thief and food glutton amongst other things. We have to lock every thing up. It's not fun living in a jail and having to carry keys everywhere. I would definately lock up your garage. If he will not stop damaging your property then it's time to lock it up against him. 

The only good thing in my story is that SS has been working and wants to work but he's already lost one job because he 'didn't learn' and couldn't get on with people. Before covid he had agency work and were on the point of him moving into a small apartment then lock down came.. No work, no apartment and it seems he's the last on the list for going back to work. 

All of these things have taken his father pushing him, him being argumentative and difficult with us and us having to force him to contact for work and all official things. He just ignores everything and would rather be on his computer or sleeping. When he was about 14 and stopped seeing is mother things started to improve, I started to see the light at the end of the tunnel. Then after a couple of years it stopped, things actually deteriorated and have got worse since. I si cerely hope that it does not turn out that way for you. Can you get any help at all?