I also think that is too little ram for your system and those settings. 75% usage means it’s going to be using the pagefile often which is bad news.
I’d try turning your settings down for now. You don’t need Ultra for 1440p IMO. If you can get smooth with High (maybe a few things that are GPU tasks in Ultra) surely that would be a good solution before spending more cash on ram?
Also try limiting frame rate to 40 or so in NVCP. This will free up your CPU and give it some headroom. You can increase it if it can handle it (with the lower settings) but 40 if consistent is plenty smooth.