Oh girl you are not alone here! 

My suggestion is to establish boundaries and rules now with your DH. 

- Skids will respect you. Hello, goodbye, thank you, etc. They don't have to like you but will treat you as an adult in the home. 

- Skids and DH are responsible for cleaning up after themselves. He can have them do it or if he refuses to parent then he does it. You are not the Maid! 

- Skids will respect your time with your DH as his wife. They will not interrupt conversations, enter your personal space, etc. 

- Your belongings are yours alone. No touch without permission ever. 

- BM is his problem. You are his wife not his mediator. He handles all things BM. 

- Skids are his kids. You can support him but that is your choice not a requirement. 

- Skid weekends are for skids to visit Dad. You can choose to participate in family activities or you can choose to do your own thing. You are not required to entertain them. 

- You are not the babysitter. Again you are Dads wife. 

These are just examples - if you start early on you can stop a lot of behaviors early on. But you need your DH to be on board. Steplife doesnt have to be hard, its the adults that can't adult that make it hard. 

Except I only have two SKids. 

I tolerate them and they tolerate me, but they are boring and ungrateful, so I've slowly disengaged. 

One of them is always competing with me for DH's attention. When I first met them, DH and I took them to a park. My two SSs went off to play. DH walked over and sat down next to me and this one stopped what he was doing entirely and stared at us. A few weeks later, we're all watching a movie at the family cabin. There are three couches, me on one and SSs each on one of the others. DH comes and sits next to me. This SS gets up and sits on DH's lap. 

The other SS is just like BM. He has a complete meltdown when he does not get his way. If DH tells him no, he calls his mother crying, saying he wants to go home. BM responds by demanding to come over to our house to see that he's ok. DH tells her to buzz off. This repeats, nearly every weekend he is with us. He's 12, so this is not age-appropriate behavior. 

Being a SM has made me wonder why anyone has kids at all. 

3 skids.  I do have bios but they were 23 and 17 at the time.  My 23 yr old DD got married and my 17 yr old son moved in with a schoolmate.

As soon as you become "permanent" (usually the trigger is when Dad's "friend" moves in with him or Dad moves in with "friend") the BM becomes threatened and the poisoning of the skids begins.  This creates a loyalty conflict and SM is instantly relegated to 2nd class citizen.  You must create boundaries asap as others have stated.  If Mr. Wonderful gets up on his hind legs about this (which is the typical NCP biodad response; this disrupts his "one big happy family model" dream) then you'll need to disengage.   

Which means: no appreciation? then no more maid/laundress/nanny duties.

Another soul mate... man of my dreams.... but..... story. 

No,  you don't "have to do their laundry".  So don't do their laundry.  No you don't have to slog through the Skid weekends.  And no.. .you don't have to tolerate their rude crap. So stop tolerating it.

The problem btw is Mr Wonderful and  his abject failure as a parent and as your equity life partner.   These kids only do what he and  you tolerate them doing.

So.... immediately implement standards of behavior in  your home and enforce those standards.  If Mr. Wonderful does not like  how you parent then he can step up and get it done before  you have to or STFU and have your back.

I'm somewhat in a similar place?

I wanted children with my partner because I actually liked being a SF, but in many ways am not really seen as a father figure to my SK15 who I came into their life when they were 12 and at that point, not looking to have another Dad (they had their BF and a previous SF before me). That said, I am getting into a place of anger and resentment because my partner changed their mind about children a year ago, 2.5 years into the relationship. And now I am somewhat confused that if we are not having children and building a family, why are we not just living like it's a party 24/7? We cannot have the expansive intimate life we could have had if there wasn't a child in the picture and I was okay with that if we were going to have more children, but now we are not.... So, why are we not traveling, have sex all the time, going out every night? Living like childless people? Well, it is because of the SK who I am not really responsible for...? I express that if we are not going to prep ourselves for children, lets have fun, and my partner basically has to reply "well I can't do that, I have a kid". It's just tough, I am bitter but ultimately cannot blame my wife. 

Not sure if you can relate anyway. Mainly where I relate is that I do the family thing, clean the house, act like a co-parent, but I ultimately am not really 100% integrated into the picture and never could be... So I want to stop playing family and just have an interesting relationship with my wife, and well the kid is stopping that from happening until they move out at 18, if they do. 

You situation sounds like mine. Up until February of 2018 I was the involved stepmom doing the same exact things you describe. I disengaged In  February for a whole host of reasons and bought my own condo. I go to my condo on kid weekends. It saved our relationship and helped us redefine the kind of life we want together. His kids made it difficult for us to be in a relationship. I have decided I don’t want to be a stepmom to his kids but I will date DH. I will marry him when his kids are out of the house and when appropriate boundaries  are apparent.  Now I have freedom to do what I want yet enjoy love and commitment from DH. I focus on my career, my social life, and I get my personal space. He stays at my house on his off custody days and I’ll stay at his house occasionally if it makes sense logistically for our plans at the time. 

You need to remember that your feelings are important in this relationship too, and you need to stand up for yourself. I never wanted kids either, yet here I am with 2 skids who never had to lift a finger to clean up after themselves or contribute in any way, until I came along. My happiness matters too, and not making skids clean up because “it avoids an argument” is not acceptable to me, much to my SO’s dismay. He could tell I meant business about having skids help around the house and clean up after themselves, so he grudgingly supported me when I made a chore chart. He usually ensures skids do their chores now. But this only happened because I forced it. I think sitting around just “accepting” the situation is ridiculous, and an easy way to make sure you are miserable and resentful.I can assure you that speaking up and making changes is worth it- things are MUCH more tolerable now. Had things continued the way they were before i spoke up, I would have been out. This is the difference between you being happy and comfortable in your home, and being frustrated and miserable. Set expectations to both your SO and skids, and enforce them. Chances are the things you need to happen will only help your skids in life anyway, so they are positive all around. Speak up!