Sadie Margaret is such a beautiful name!
I probably wouldn’t use Sylvie unless this going to be your final child and you want matchy, as it might commit you to a S - ie name down the track. Too similar for me!
My daughter’s first MN is Frances - I love it!
Hazel Frances is so pretty! That would be my pick.
Or else Mabel Frances. (I do love Mabel, probably marginally more than Hazel - though some might say Sabel and Mabel or Sadie and Mabie)
I love Rose but don’t think it flows as well to Pearl or Frances. So if you want to keep Rose, which is one of my favourite names, I think Rose Francie (to lose the repeat S, and a nice repeat of -ie of Sadie) or Rose Francine.
I think a longer first name flows best into Pearl - eg Abigail Pearl, Josephine Pearl (but maybe Josephine too stuffy with Sadie?), Claudia Pearl, Matilda Pearl, Lydia Pearl, Avalon Pearl, Tallulah Pearl (love this with Sadie Margaret!), Harriet Pearl (Hattie), even Frances Pearl nn Francie would be very charming!
On the other hand, Margaret means Pearl so are we sure we need to double up on the meaning - ie could Margaret stand in for Pearl as well? Could you use Greta as a first and that be a double down on Margaret/Pearl? Eg Greta Frances.
Otherwise: Eliza Frances, Elsa Frances, Norah Frances or June Frances.
Anyway, would love to hear your thoughts - you’re definitely on the right track! Hazel or Mabel would be perfect, and Rose with some tweaks!