Looking for refresh suggestions

Need help and am looking for suggestions to upgrade my wife's Ghost S1. She recently upgraded to a 4k monitor and the CPU seems to be a major bottleneck. Any suggestions? Haven't built anything in a while but willing to upgrade pretty much anything besides the GPU and case, she loves the case and a 4090 or 5090 isn't in the budget.

Current specs:

• Intel i7 8700k (Unlocked) w/ NH-L12 CPU Cooler
• Nvidia RTX 3080Ti FE
• ASRock Z390 Phantom Gaming-ITX/ac
• 32gb 3200 DDR4 Vengeance LPX
• Samsung 970 M.2 1Tb
• Corsair SF750 PSU