Realistically how often do people say full names out loud!
Sadie Mary *surname* I think would be fine!!!
My DS2 has a middle name that doesn't flow as nicely as my DS1 first and middle name but it has huge meaning to me do I used it!!!
I think the importance of passing on the name trumps it doesn't sound 'quite right' with Sadie!
I don't like Eilish purely based on the fact I have NO idea how to pronounce it and wouldn't want to burden my children with a name that is constantly mispronounced, they have to spell etc etc