Don't get married.

n/m you already have a kid with him.  Leave all the parenting to him.  Read a book about boundaries, his kids and his family are his responsibility.

So when I first came here I thought disengaging meant shutting down everything involving the steps. That isn't always the case

You and only you control your disengaging. You can simply chose to not parent or discipline when they are around . You can take your bio elsewhere when they are over. You can ask your spouse to do things for his kid you'd normal do.

The possibilities are endless. But it's important you are in control and do what makes sense to your situation and you. 

For me this meant :

- pointing things out to DH to tell the kids ( stop leaving cherry stems on the table, don't leave clothes on the bathroom for etc.)

- not discipling unless I absolutely needed to ( I had to yell at SD7 to stop making annoying loud noises when I had a headache and DH stayed quiet )

- allowing kids on trips out I invited them to only. I don't care if SD wants to go to the gas station with me . If I don't want her to go I don't let her. But I will plan trips out with her to the mall etc. 

Be your own guide and follow your own rules. If your DH asks you can either explain or let him do the math.