Let me explain. For higher frame rates you need faster CPU cores. Not more cores or less cores but faster CPU cores unless there comes a game that needs 8 cores/16 threads. Now whether that extra speed comes from higher clocks or better IPC doesn't matter as long as it's faster. Ideally you want both faster clock speeds and higher IPC. You can probably get frame rates higher than 60fps and higher than 120fps. A consistent 180fps probably isn't happening but I'd say even higher than 144fps is possible with your processor. Now you need to ask yourself if 144fps is enough for you. A way you can find out is to check how many hertz your monitor is. 60hz, 120hz, and 144hz are all fine but 180hz is something you might consider upgrading your cpu for. Now if you had a 180hz monitor I am not sure if you could even feel the difference from 144hz to 180hz. So it may even then still not be worth upgrading your CPU for. I see you plan on upgrading to a 4K monitor. Don't forget you can also use a 4K TV as a monitor if you can find a good deal. A 4K 32 inch TV would work nicely as a monitor. With 4K you're talking about lower frame rates and lower cpu usage. At 4K the gtx 1080 ti is working harder than the CPU. So at 4K you don't need as much cpu power. The reasons you would want to upgrade to an 8th generation cpu is for the extra cores which you already have 6 cores, extra features, lower power consumption, and higher overclocking potential. All stuff which you could probably make it comfortably without having.
Short answer.
You can max out every game at 60FPS with your CPU at 1080p or even 4K. You don't need an 8th generation CPU unless you want extra features, lower power consumption, or higher overclocking.