At 1440p the difference in fps when upgrading cpu (from 6700k) is too small to notice (just google benchmarks), therefore all that money will be wasted.
Just buy a used gtx 1080 - good price, on par with 2070.
For CPU, why not buy ryzen 2700x? That CPU is just too good compared to any intel CPU in that price range, and even beyond that price range. (same performance as any i7 at 1440p (maybe 1 or 2 fps difference), and you get 8 cores 16 threads).