For the stacking, on your tablet breakpoint (or wherever you’d like to stack the columns), switch your flex direction from “horizontal” to “vertical”, as shown here:
For the white margin on the map wrapper, remove the max-width altogether from your map image, and instead give it a max-height to control it from getting too large. Around 500px looks about right.
@basecamp59 Same concept! Find the parent of that list element called “Columns 3” and set the flex direction to vertical instead of horizontal at the appropriate breakpoint.