Cpu or gpu upgrade

So my monitor is a 3440x1440p (but I often play at 2560x1080p due to the low performance of my current GPU / CPU). Since at the moment I can't afford to change all the pc, which one should I focus on it first? New CPU (and Mother board+RAM): Ryzen 5 3600 = about 400 EUR / 440 USD For the GPU...

I'm looking to get an acer predator x34 i have a evga gtx 980 ti. ill be up grading the card to what evga puts out this year but i was wondering if it woudl be a waste of money. i play league mainly but ill be palying some fps games like destiny 2 and what not