Oh for God's sake, people really overthink this stuff. Google treats hyphens and spaces like they're basically the same for most common phrases - the volume difference you're seeing in your keyword tool is just how they bucket the data, not some secret ranking signal. you don't have to pick a side and fight a war over it.
Just write whatever reads naturally for humans. Stick the variant in your title tag and H1 if you want to chase the higher number, but don't force some awkward version into every paragraph. The body content? Nobody cares. Google already combined them at the index level half the time anyway. Check the actual SERP - if the same pages rank for both spellings, you're done.
Tools love splitting volume across variants to make you think you have a decision. it's a micro-optimisation that matters less than your dinner plans. Depth of content, internal linking, actual authority - those will move the needle. "Pop-up shop" versus "pop up shop" is not why your blog flatlines. write naturally and move on