My name is
Emma Rose - a good ER combination :P
But seriously, I think it would be absolutely fine to go with different initials for this child. I don't particularly like it when the whole family is matchy-matchy, and it might mean that you end up using a name you don't even like, just to fit the pattern which is silly.
Also, you only have 2 children with matching initials so it won't be really obvious that this one doesn't match. If you had 15 children when ER initials, and then one without, it might be a bit more of a problem.
I like all the names you suggested, especially
Amelia,
Susannah,
Thomas and
Elliott. I think it would be nice to use
Michelle as a daughter's middle name, as it is related to
Michaela.