Maybe I'm reading the wrong shelf, but the whole galactic soul idea seems more like a vibe authors chase than a fleshed-out theme. It pops up in space opera where characters feel the 'music of the spheres' or whatever, but it's often just backdrop. I keep thinking about 'The Left Hand of Darkness'—that book gets at something like a shared, planet-spanning consciousness through the Gethenians, but it's cold and anthropological, not cosmic. Then there's 'Solaris', which is literally a planet that reads human souls, but it's so alien and melancholy it hardly feels galactic. Most stuff labeled with this is just romance with spaceships, where the soulmate bond glows with nebulas. I want the philosophy, not the aesthetics.
Funny enough, the closest I've felt was in an old short story collection, 'The Stars My Destination' maybe, where the protagonist's rage feels bigger than space. It's not peaceful or enlightened, but it's sure as hell a soul expanding past a single planet. Maybe we need to stop looking for harmony and look for the ruptures.