When my lifemap was loading today there were a few seconds were the traces of all my runs were visible but the underlying map hadn’t loaded yet so it was just the LifeMap of all my runs on a flat background. I thought it looked really cool, seeing just the run outlines with no underlying map for context.
It’s probably not worth cluttering up the UI with another button to turn the map background on and off on the LifeMap screen but I figured I’d throw the idea out there.
In case this never makes it into CityStrides, you may want to check out the below. I’ve used it to create screenshots of exactly what you described; there is a toggle option to turn off map background. Also used to create a video of heatmap of an area.
Note: I don’t think the developer is actively working on site so it is a bit clunky and slow so may take a minute to load.