I would suggest get those 1080 ti before they extincted, I mean production for it has been stopped so yeah. And then don't get a 2070 because it didn't even beat the 1080 ti. Unless again the 1080 ti are out of stock
This is quite tricky, your system is already very good and balanced and indeed to get more you'd be looking at lots of spending.
Given how you want to use your current i7 4790K as a dedicated streaming PC I don't see a point on getting a Ryzen 7 2700X since it's deal is mainly being able to stream. You want refresh rate gaming and for that you'd want the i7 8700K instead, which still is by all means a "workstation capable" CPU.
Now for the GPU the RTX 2070 is a GTX 1080, that is just 1 single tier up from your current GTX 1070, if you're going to spend all the money on something new I'd honestly try pushing it to be as meaningful as possible with the GTX 1080 Ti or RTX 2080 coming in mind.
