Stay AMD imo. 3900X is unnecessary for gaming, if you want to dabble with Adobe After Effects or some other workstation task, go for it, but for gaming, your 2600 should last a little longer. 2070 Super is also excellent, even though the Ampere generation is absolutely insane. Just know that from what I saw on the spec sheet(but the information can be a little elusive at times), the MSI B450 Gaming Plus Max doesn't have PCIe gen 4(as most B450s are), so you won't even get the full benefit from upgrading to an RTX 3070. Keep in mind that maybe waiting for RTX 40 series is a viable option.
If anything, a Ryzen 3 3300X will be a very cost effective upgrade. I heard it's currently out of stock(depending on where you live), but when Ryzen 4000 drops, it should become available. Can't really get a Ryzen 4000 on a b450 mobo, but you should know that the 3 3300X was actually released just 3 months ago, so it isn't really a Zen 2 processor, more like a Zen 2.5.
View: https://www.youtube.com/watch?v=vD8Yk7JrBL8
Would recommend waiting for a Ryzen 5000 with RTX 40 series(or whatever AMD has to respond to the 30 series). Ofc who knows who will be the market leaders by then. The games you listed above aren't very intensive, so if your streaming, just get a better CPU(like a Ryzen 7 3700X or even just a Ryzen 5 3600X) to handle the added load brought on by streaming. Maybe your GPU is already good enough to offload the work from your CPU. I run R6 @ 1080p, max settings at around 134fps uncapped on an RX 580 4gb, so a 2070 Super should crush everything you seem to want out of it.
Just FYI, I'm just all about value, that's all I care about, so keep that in mind when you read what I say. I'm just a teenager, so I try to stretch the little money I earn from my part time job as well as I can. Idk, maybe you can find something alluring on the used market. As a bargain hunter, the used market is my best friend. That's where I got my current gaming rig, bought it for half the price it was originally bought for less than a year ago, with a 27" monitor tossed in.