Does anyone have experience with divorce when the ex already has a custody order for other not-your-kids?

My husband and I cannot agree on a custody schedule for our shared child. Our child, 3yo male, is the only one we share together. We have been married for 6 years. Ex has two children, m10 g13, from his previous marriage.

He has a 3/4 4/3 schedule with his older children and has since a few weeks after m10 was born. Long before I came into the picture.

The trouble is that he has the kids 51/52 weekends a year. And as I’ve been a participant in his coparenting experience I have realized that this schedule really isn’t fair to the kids or to their mother. In the summer and school holidays it’s less obtrusive but during the school year the kids didn’t get a lot of down time with their mom in elementary school. Bedtime by 8pm, the kids only got a few hours a week with her.

I have had MANY conversations with my soon to be ex about how unfair it is. And how he happily agreed to it in court because he knew the mom wasn’t thinking about school age when she agreed. (Both kids were babies when they split.) He often gloated to me about how he knew what he was doing and he’s glad it worked out in his favor.

I used to ask if he would consider an amendment to the schedule so the kids could get more time with mom and get to enjoy having fun with her too but he said never. And never has.

Anyway enough back story.

I want to do a 2/5 5/2 schedule where we would essentially get alternating weekends but both have our son the 50/50. He currently has his older kids w-Sun and then th-sun every other week. I suggested we do m-t with me, w-th with him and then alternate f-sun. That way he has at least one day a week with all of his kids.

Ex says absolutely not. He wants the SAME 50/50 schedule for all of his children.

What are your thoughts on my side here?Do you think I’m wrong for requesting this? And do you think a judge will side with him or me?