Hi all, incase anyone is looking for a fixed background over an entire page with one of the built-in image effects, here's how I cobbled a solution together:
1. Moved all sections together into one single section with the section at the widest width.
2. Added code and spacers to skinny up a few of the blocks that hadn't been the widest width.
3. Added this code to fix the background's position:
** I chose to apply the background without code to utilize squarespace's responsive cropping. I like the focus feature personally. Applying the background this way also allowed me to use a built in webGL filter for some extra razzle-dazzle.