That isn't really too suprising, the 960 really is designed for 1080p gaming, not higher. It's also maxes out on vram at 720p ultra.
I was actually surprised more in how well it handled 3440×1440.
If it hadn't been for Horizons I would been perfectly ok with it.
And as mentioned the main bottleneck was VRAM for that resolution.
And this was during the 2.0 beta. The level of optimizations for planetary surfaces have come a long way since.
A 4GB 960 would have probably done ok.
A 1050ti with 4GB should actually do a little better.
Of course to knock it out of the park you could go for a 1070 but that would also mean needing a beefier psu, and higher demands for airflow and cooling.
I ended up quickly upgrading from the 960 to the 980ti myself and I had add at least one case fan to get running temps down from 75c and out of the realm of possible thermal throttling.
If budget is tight, I stand by the 1050ti recommendation, but yeah okay higher than 1080p you might not max all settings, but the game look more than ok at even medium.