I still feel like the 5DSR is about a stop worse in low light than my 6D was. For me the issue was less about the random noise associated with high ISO than it was about purple color blooms and lines I was getting in the darkest areas. But that said, I had no trouble getting night sky images that I really liked with the 5DSR.
I think you are on the right path. No camera/lens is going to take a great photo where there is no light. If you want detail in the landscape then you need light. Options for that include taking a separate shot in twilight for the ground, adding light yourself, shooting with some moonlight available, or doing a really really long exposure to collect enough light from the stars. Jim and I both seem to be in the camp of trying to time our night shots for when there is a crescent moon available to light the landscape. Next Saturday, for instance, looks like a great opportunity and I plan to shoot the Milky Way over Hidden Lake in Glacier. If I have clear skies and can avoid a bear attack it should be fantastic.
Fast glass really does help. Going from f/4 to f/2.8 saves you a full stop of ISO noise. With your current gear, I'd try to make use of that 24-70 a lot, especially since I like the way the MW look at 24mm. With a small investment, the Samyang/Rokinon 14 f/2.8 does a really nice job for wider angles, although for a little more money (should the bug really get you) the Rokinon SP 14 f/2.4 is a much nicer lens and for a lot more money the Sigma 14 1/8 that Jim uses gathers a huge amount of light.