You're not going to notice much difference between CPUs gaming at 4k since it's much more GPU bound anyway. So you'll have a similar experience whether you have an 8700k, 7980xe, or 2700x.
For streaming, anything more than 4 threads would be fine really. Older i5's are 4C/4T and can struggle. Newer i5's with 6C/6T can handle streaming alright, but Ryzen with 6C/12T+ or newer i7's with 6C/12T+ are ideal. Older i7's with 4C/8T are still solid CPUs though, and since you have a Kaby Lake i5, you might find a simple CPU swap to an i7 beneficial and much less expensive than a socket upgrade.
Also, keep in mind for streaming that typically with an Intel CPU, you will have a better experience in games, whereas with AMD Ryzen CPUs, viewers will have a smoother experience.
Editing is where you're going to notice most difference. Whether you go HEDT or not is entirely up to you. If you're not editing/rendering large high resolution videos on a daily basis and maybe even making money from it, I probably wouldn't recommend HEDT, unless you're an enthusiast. A Ryzen 7 CPU, like a 1700(x) or 2700(x) would be a solid option IMO if you're looking to upgrade socket entirely. Otherwise, swap your i5 for a Skylake/Kaby Lake i7 and you should be good.