To eliminate wonkiness, I would try using a walking foot. Also be sure the machine tension is good. I would try sewing the whole thing in strips, but since I've never tried something quite like that I might run into issues like you did. It's harder to work with bigger pieces of fabric and longer seams when you're used to making blocks!
If you want to make blocks with the resulting colored strips being 6" wide, the block's diagonal will be 24". The pythagorean theorem says the block's finished side is then 16.97".
To make blocks, I would still start with a strip set, so I'm not sure of the purpose of cutting it into pieces and putting it back together, but that is what quilters do!
