I think Jasper fits best with the names of your other children. I also like the Danish form Jesper. After Jasper, I would pick Daniel.
I like Oliver, but it doesn't fit with the names of his potential siblings. Phoenix is too "out there" compared to the others; if you wanted to go with something edgier, I would go with something low-key. Phoenix is very bright and shouty. I don't like Jacob -- could never get past the "cob" part, not to mention the fact that it's one of the top names in the US right now. Ayden is so trendy looking ... Aidan would be more acceptable. Same with Caden, but that's even more trendy and flash-in-the-pan.