I like both Greta and Maren! For boys, Cole. FWIW, I haven't met any preschoolers and younger with that name. I agree that Evan sounds too similar to Vivian. I like River, but it feels a bit out of place with the siblings' names, if that concerns you at all.
Greta and Bryce. I would pick a different ending sound because I would trip over 3 names with -n endings.
I don’t care for the name Cole, it makes me think of lumps of coal. I was at my son’s soccer game yesterday and cringed every time a Cole’s parents cheered for him.
Re: help with 3rd baby name
I like Maren and Greta. Greta is so unexpected.
I like Cole and Bryce.
Suggesting:
Miriam
Brynn
Cora
Delilah
Gretchen
Julia
Leah
Rose
Clara
Georgia
Alexander
Benjamin
Elijah
Ryan
Carter
Grant
Jack
David
Shane
Travis