Hi,
So, where your sub is placed relative to your listening position matters quite a lot, both from a room harmonic order standpoint, SBIR standpoint, but also just the SPL itself. The farther you are from a speaker, the higher the SPL has to be to be at the target SPL at your location. It falls off by -6db every time you double the distance. So if you move the sub away and want to hear it at 75db SPL you'd have to increase the volume the sub, as it goes farther away, to make that happen. This means everyone else gets to hear higher SPL too. If you move the sub to be right near you, the distance is much less, and so less volume from the sub to reach the SPL you want at your listening position. So you get the same SPL with less power. That means your neighbors hear less of it because they're farther away from the sub than you.
I would not lower the volume on your surrounds. Instead, use placement. Imagine 30 to 60 degree cones coming from their faces. Place them so that both seats are within the 30 degree cone if you can, or at least within the tightest towards center of a 60 degree cone at the least. Keep them at ear level.
Your atmos situation is dire. Your only option is stands or reflection based stuff. You can either get creative about how to get speakers on the ceiling/wall boundary like heights, or you can just keep doing what you're doing. It's understandable that you cannot cut holes or drill. That's fine. Sometimes there's just no option when compromising. You can either try to make the most of up-firing reflection based atmos effects. Or you can try to get creative with placement of those speakers higher on the wall and aimed at your listening position, like height speakers. It's not the same thing as atmos. But then again, a single channel up firing doing atmos is also not really atmos. It's just.... different. You either like what it does, or it doesn't do much. That's up to you to decide.
Very best,