I'm sorry you are feeling this way during the holidays, they often seem to magnify the negative feelings. I think on some level you realize the truth - you have a DH problem way more than a SS problem. Your DH is not parenting his child and you are seeing the results. It is time for a "come to Jesus" conversation with your DH where you tell him exactly what you are feeling and what he needs to do to fix things.

A hill worth dying on is the way your SS treats your dogs. If he gets bit, your dogs are going to suffer the consequences. Do whatever you need to do to protect them, even if it means rehoming them on even a temporary basis. They only have you to protect them.

Worn out, worn down by step life. I never thought it would be this difficult. These children hold so much power in the dynamics of my marriage. I have yet to overcome the triggers of the thoughtless behavior of snotty entitled brats, twisted round their mother's little finger. It's all so warped and I'm at times convinced these children create some of these situations intentionally. Before DH met me his kids picked what they ate, watched, where they went etc. And yeah I had a big part in putting up boundaries, but now he can afford a house instead of a room... I mean it was that ridiculous. So I'm the target for all the anger because ss and sd don't get catered to anymore. Don't get me wrong, they have all they need and then some but now we pick meals, spend less on entertainment, don't eat out as much but in turn we have money saved for the future. These kids don't give two flips about our future. Plus their BM hates me and children internalize those feelings from their mom. It's a losing battle and the way they both parent, these kids won't launch. I suspect after ss17 ages out and bm doesn't get child support, it will be our problem. My marriage will not last that. 9 years now and we still get in arguments over their behavior. I have never seen DH disciple or get angry at them. I get triggered by something they do and DH will get mad at me for being triggered. I'm stuck in an endless loop here, I feel I'm also at a breaking point. It doesn't end at 18.

My Ss17 lives with us full time, and has not stayed at his moms for a few years now. She's homeless and has some mental health issues. Her parents passed away and she doesn't speak to her siblings so there is no support from that side of the family. DH's entire family lives out of the country so there's virtually no one else that is family that is involved with these children (Ss23 moved out earlier this year). 
Ss17 is a huge liar and has been since I met him. Actually they both are but since Ss17 is the one who still lives with us he's the one I'll focus on now. He followed the same pattern your stepson is following now. 
Most of his lies now are about school. He lies about going. I very rarely ask him much of anything because I don't know that I truly care. This year, I had noticed that he kept towels by the crack of the door and I told my H that he was probably smoking but H said maybe he was just cold. 
Turns out I was right, kid is a marijuana smoker now and says he has been since 4 years (kinda in a mean way to let us know we didn't know what was happening in our household). 
A few days ago, Ss17 was returning from work at his very per diem job at foot locker (which on day 2 of working he reported as being the best salesperson), and it was about 10pm. I was sleeping so DH picked him up and told me what happened the next morning. Apparently DH told me he called him and said he had been hit by a car. I asked DH how it happened, and he said he got hit  when he was on his skateboard. One of SS17's lies a couple years ago was saying he fainted on a New York subway and when he came to he was sitting next to a stranger (his excuse for not going to school that day). His lies can get very extreme so I wondered if it was simply that he was high on his skateboard and fell but instead decided to say he was hit by a car for more oomph. 
As DH is retelling this story, I asked if SS was okay, did he hurt himself and he said luckily no, because he had on multiple layers of clothing, but he did state his skateboard was "broken in pieces". Immediately my guard was up. I asked DH what makes him believe SS was telling t he truth and he immediately became defensive, stating that he saw SS had a swollen knee because he checked him out to make sure he wouldn't have to take him to the ER. 
He allowed SS to stay home from school. When I returned from work, SS skipped down the stairs with no limp, nothing. I casually asked about his hit and run so as not to seem cold, and he confirmed. I asked if his knee was still swollen and he told me no (most likely because he knew I'd probably check). 
When I looked at the skateboard, it was broken, but in one place. 
I myself am on the brink of wanting to separate from DH due to SS. I really don't like his kid as a person but I understand that it's really the parenting and lack thereof that caused them to be unlikeable people. And if it wasn't for my daughter, I don't know that I would have stayed with DH. 

every day for the rest of your life.

Yes? No?

That is the question. If no, end it and get on with your new life adventure and leave this tragic failed gene pool behind.

Take care of you.