Games will be limited by the cpu or the graphics card.
Which will depend on the games you play.
There is no such thing as "bottlenecking"
If, by that, you mean that upgrading a cpu or graphics card can
somehow lower your performance or FPS.
A better term might be limiting factor.
That is where adding more cpu or gpu becomes increasingly
less effective.
Try this test:
Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
If a graphics card upgrade is indicated, a GTX1660ti is only a modest upgrade.
You might want to go stronger or you could be disappointed if you do not see impressive results.
If you think cpu is your limiter, see of you can overclock your 4690K a bit.
Yes, dual channel ram is a bit faster, but not significantly so.
Past that, adding another 8gb stick may have compatibility issues.
A big boost to processor capability will involve an upgrade to ryzen or 8/9th gen intel.
Either will also need a new motherboard and ddr4 ram.