OK, good. I don’t see how to accommodate the sts or join the back held sts to the sleeve without a circular.
You want the split up the back yoke to be between the 21 and 24 back sts.
As an alternative, you could continue on the straight needles and work those 21sts with a separate ball of yarn creating an opening between them and the sleeve which would be seamed up later. You would still turn at the split between the 2 backs but some of the sts would be worked with the second yarn ball.
Your knitting looks very pretty and it’s a gorgeous dress.