Semi-professional SketchUp use...in terms of ROI, will most certainly will pay for that GPU upgrade over the next few months anyway. I have one design lined up...maybe another.
I'm still on the fence...leaning toward the Vega since I use two displays and I like a fast computer for work...gives me more time to play music. Ideally I would buy an iMac Pro, but no way I can afford that at the moment. The GPU just died on my 2011 so I need a new computer now...I was planning on saving up for an iMac Pro, but to be honest I think the 2019 will be totally sufficient. That said, "The Vega 20 is still a good bit slower than the 580X" has me leaning back towards the 580X, that MacBook Pro I borrowed was well fast...although I never tried it with a second display.
After reading this, I've flipped. I think you should go for the Vega actually.
Now if the interface is entirely static, having to run more pixels won't necessarily strain the GPU a lot at all; But it will fill the memory in the GPU more.
The reason I'm now leaning more to the Vega for you, is that you'll actually be able to earn a bit of cash from the work the computer will facilitate, work in which the GPU will play a significant part. If the faster GPU choice will mean you can make money faster then that's essentially increasing your hourly salary through a one-off investment.
The 580X is as I mention a fair bit faster than the Vega 20, but the 5K iMac alone also has almost 3x the amount of pixels. Now as mentioned that doesn't necessarily scale with performance, especially for static content, but if you're working with content in SketchUp and SketchUp renders based on screen resolution that is a lot more to drive regardless.
Initially I figured you were mostly into music production and just did a bit of SketchUp for fun, but it now sounds more like you do computations that could benefit from the extra power, and lower temperatures.
Now if you work with high resolution textures, the Vega also has another feature that's a massive benefit; HBM memory.
The benefit of HBM2 over GDDR5 is very workload dependant, but especially beneficial when working with high resolution, high detail textures and a lot of pixels.
As someone else mentioned, the lower heat produced from the Vega will also help the lifespan of the computer, and help it stay performant down the road when dust may inhibit the cooling capacity somewhat.
I would not advice most people the Vega, and thus didn't initially, but after our messages here; I feel you'd be better off with it actually. I think you'd regret getting the 580X when you have your first GPU related slowdown.