9900K? I would consider if its cheap enough. If prices are worse than a used Zen 3 get Zen 3 instead. If 1080p or 1440p a 5800X3D is the only answer for a good budget. AM4 lets you keep your old RAM, sell the rest and get the 3D. Otherwise a 9900K can be good if its cheap enough, you will need to overclock it but on a Z board 5GHz+ should be no issue.

But don’t pay 250+ € on a used outdated CPU or you’re dumb. Thats almost the new price for the 3D and its superior in any way. The 9900K was and still is somewhat good, but it will have higher power draw, requires OC and lacks PCIe 4.0. So I would only get one if the price was really good.

And the 4060 Ti is a pretty bad deal, depending on your price I would return and get a 4070. Much better price/performance and 12 GB. You can keep using your CPU even if it bottlenecks a bit, a CPU bottleneck is always preferable to a real GPU bottleneck: GPU = always low FPS; CPU = occasional drops under high CPU load, min-FPS spikes.

The 3D will of course work wonders especially on min-FPS. I would watch Youtube comparisions like this one: https://www.youtube.com/watch?v=6UAES7F48EU

40 % from a 9900K, so for you it will be a bit more. But keep in mind, in 1080p or 1440p. According to this test it was just 16 % in 4K. For 4K you definitely don’t need a top end CPU. And the 5800X3D delivers almost Zen 4 performance but on the affordable AM4 platform.