SVG has a feature that allows grouping plot elements. This g tag can have its opacity adjusted, which is different than adjusting the opacity of the individual elements as demonstrated in this image from this answer where there is more explanation:
Not all graphical back-ends support such a feature, but when the back-end does, it is likely more efficient than computing the geometry of the union (though, if you want to e.g. add a border around the union, you’ll likely need to compute the geometry anyway). Do either Plots.jl or AbstractPlotting.jl/Makie support this?