I also would go with a 9900K or just leave it be and wait for next gen. There's not enough performance increase in 10000 or 11000 series to warrant the cost of a new board.
Also the truth is any high end CPU from the past few years will generally be enough to get you to be GPU limited, especially at 1440P or higher. Even an 8700K will do that, or for that matter even a Ryzen 3600--not in all situations but in most situations. That's another reason why it's not worth buying a new board versus simply upgrading to the limit of your current one: you don't need the absolute fastest processor on the market, just a fairly fast one.
That being said, if you only have a 1080 you're probably GPU limited most of the time even with your current processor. If I were you I'd prioritize a GPU upgrade over a CPU upgrade (granted GPUs are not the easiest to come by right now).