A whack with a hammer will flatten them as well, after they're sewn. Sometimes we just have to do what we have to do :)
I've made a Giant Dahlia which is 16 seams in the center sewn together. I ironed them open as much as possible, then I was able to sew across a flatter seam. It's tricky, but can be done.
8 seams won't be perfectly flat...just know that. There will be a hump, but the flatter you can get it, the better it is. If you have a picture of the block, or a link, that would help in some specific ideas for you.
Debbie in Austin