Having gone through this process from a 2013 MBP to a desktop 18 months - 2 years ago and now making another shift back to a MBP, I thought I would share my thinking with you from both a hardware and software perspective since my use case sounds almost identical to yours.
I made the decision to shift from my MBP to a desktop in order to use Vray for Sketchup with some sort of performance but also to process photogrammetry datasets from my drone surveys.
Apple didn’t make a machine that could do that at that point and I did not want to lock myself into an upgrade deadend with an iMac (and also not having a large budget).
I decided to go and build a Hackintosh with an Nvidia card (1080 Ti) which would allow me to benefit from GPU/Hybrid (both interactive and production) rendering in VRay and also massively boost the photogrammetry performance via CUDA based GPU compute on the card. I chose a 6 core Coffeelake i7 8700K 64Gb RAM and 1TB NVMe SSD and (mostly) all was good. Performance was (is) great, it ran rings around any Mac on the market at the time (at a fraction of the cost) and I got GPU rendering and CUDA (once you have GPU rendering, you never, ever want to go back to CPU). It did, however, mean that I have to stay on High Sierra as Nvidia and Apples’ spat meant that Nvidia stopped making web drivers for their cards from that point onwards. (all the bits cost just under 3k Sterling)
At that point in time OpenCL was being deprecated by Apple as was OpenGL, Vulkan wasn’t mature, and Metal…well. So CUDA was the only GPU compute in town and I am a VRay for Sketchup user - the integration is awesome, I use the asset manager and VRay Tools as part of my SU/Layout construction documentation workflow even if I am not rendering in Vray.
Things have moved on a bit, AMD looks like its getting back in the GPU game with both feet with Navi and the forthcoming Big Navi, Metal compute shaders have matured, realtime engines are where its at and after 18 months and lots of consideration about where to go next, I think its worth separating SketchUp performance requirements from Vray / Other rendering performance requirements.
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SketchUp Performance
SU is single threaded as is Layout, on my 3,6Ghz Coffeelake that single CPU core is rarely 100% utilised, usually it will be Skalp generating rear-view projects that mostly utilise it.
As for GPU, most of the time I don’t think the 1080Ti realises SU is running, looking at the GPU load it is mostly well under 50% - If I have a big model open with all the 4k textures then it can spike to maybe 60% and that is where lots of VRAM is beneficial.
Main memory headroom is where I seem to suffer with SU, even with 64Gb Ram occasionally I still need to manually manage memory purges to provide overhead with several large model files open.
Your maxed out iMac is going to give you way more bang for buck than the base model MacPro but you are locked in to that config (sort of - eGPU not withstanding) and SU probably wont even touch the sides of the capability of that machine.
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VRray performance.
To cut to the chase, because Vray doesn’t support GPU rendering on anything but Nvidia GPU’s, the GPU in your hardware is not relevant, so the performance question is purely down to CPU core count and on Apple’s price equation on the iMac Pro that equates to 400 dollars per core increase over the base spec (so 8 core to 10 core = 800, from 8 core to 14 core =1600 etc). I am not sure that provides good value in terms of raw (rendering) performance over that 1600 dollars spent on a GPU (if it were currently possible to run an Nvidia card on a Mac - that 1600 buys you a RTX 2080Ti with change).
Understanding V-Ray Hybrid rendering | Chaos
TBH I do most of my production renders on Chaos Cloud now - its pretty darn good value and my machine is not tied up rendering and I can carry on and earn money working on another project whilst it does the job in the cloud. Obvs where the GPU is beneficial (in my case) is for interactive rendering during look development etc.
But. I need some portability, I still want to use VRay , although I am moving towards realtime engines - ultimately Unreal once the Datasmith plugin is available for MacOS 
but for now mostly TwinMotion.
Also, I would like an OS upgrade path, (as I said, I’m locked to High Sierra), I don’t want to move to the Windows platform as my primary OS, I’ve been a Mac user for decades. I would also like some hardware upgrade path and I think, from the ways things are going, the most important upgrade element will be the GPU.
So my current plan and thinking is to plump for a maxed out 16" MBP in October / November when the refresh is released with whatever CPU bump we get and whatever GPU bump (probably the M variant of the 5700 - maybe a nice surprise with the M variant of Big Navi if it gets announced in September but I doubt it. I would then add an eGPU enclosure to give me an upgrade path in terms of Big Navi or if hell freezes over and Apple and NVidia play nice again. This would be more than enough to run SketchUp for years to come
My current Hackintosh I would then turn into a PC to run as a Vray render node, the photogrammetry stuff, any element of my workflow that currently is difficult / impossible under macOS and as a bonus run VR hardware for SU viewer and Twinmotion VR modes.
Mac users are in a hard place when it comes to rendering and Archviz, on-one really uses OpenCL anymore, CUDA hasn’t been updated since High Sierra and Metal Compute Shaders haven’t been adopted really outside of Apple (although Adobe just made an announcement last week that they are going to optimise their apps to take advantage of Metal acceleration) Chaos Group are understandably firmly wedded to CUDA currently - I was on the beta for Vray Next and one of the techs said they looked at Metal for raytracing but it just wasn’t viable or mature enough at the time.
I am optimistic for the future however, Metal seems to be getting mature, AMD are getting back in the game with Big Navi and raytracing, Apple’s Intel performance offerings are getting better (although the thought of a 64 core Zen 3 is mouth-watering), so things may be coming together.
Sorry for the ramble but these are just my current thoughts having been around the houses a few times - and may not be considered ‘help’ in the traditional sense 
but it might provide some alternative perspective.
ThreadRipper builds that are very very easy to get working.
