I guess for this simple case, AlgebraOfGraphics should just offer a horizontal legend switch. I like it a lot that you can pull out the legend, move it, and restyle it, but of course that adds some code that you don’t want to write everytime.
Regarding the spacing question, that exact case is talked about in the layout tutorial https://makie.juliaplots.org/stable/makielayout/tutorial.html#Fixing-spacing-issues
You can trim away the now empty space and make the legend adjust its own row height down.