Might be worth looking at a PC instead... There are areas that PCs are just quicker and better bang for buck. That 5k display has a lot to answer for too 14mps 60fps any high end card is going to struggle for a smooth experience.
worth a read
https://www.pugetsystems.com/labs/a...2018-iMac-Pro-Mac-Pro-vs-PC-Workstation-1159/
The above to keep it fair ran metal vs open CL.
Its also worth noting that yes you can add another GPU via TB3 but it doesn't mean apple will support it, there will be no official CUDA support which is important depending on what your doing. CUDA outperforms metal in quite a few area. There will only be support for cards that have drivers.
There's no point in buying something and hoping support is added later.
Its also a fact that you need to add £400 to your GPU price if you do buy another because you need an enclosure for it. Adding 50% the cost to a card then only get 90% of its performance is a poor prospect IMO. Putting a card in a tower and it being out of the way and running at full speed is a far better idea imo.
The nice thing about the PC is that for the time being and it has been this way for a while is that CC runs better on selective hardware and apple doesn't give you much option like only ATI cards. Instead of £800 for every upgrade with the PC you can add hardware as you need it without the upfront cost. CC's code isnt great and hopefully will improve and if apple are serious about getting back into the pro field hopefully it will improve.
Need to evaluate your needs and get some more information before considering spending that sort of money and being disappointed.
The other thing worth mentioning is that CC is cloud based. The UI is the same and is essentially the same experience running on mac vs windows. I swapped 2 years ago and I continue to use a mac for casual use but at work everything is now running W10.
I hadn't used it since XP and it did take me a week or so to get used to it but... it does a lot really well and with HS being so poor on launch haven't really looked back. I was interested when the iMac pro came out but just hate the fact its an AIO.
Programs like after effects hare hardware intensive and it being an AIO really doesn't give you the best longevity regardless of the specs, in a year they will be outdated and apple in its infinate wisdom is unlikely to update this thing. Look at the mac pro... 5 years.
On a PC if better hardware comes out thats worth upgrading to its as simple as swapping the MB and processor out, or stick a new GPU in. What I used to do with my 5,1 mac pro for years as you see in my sig, apple doesn't offer this anymore and as impressive as the iMac pro is... your stuck with what you buy. Its incredibly difficult to upgrade and although you can upgrade it... how much will it cost and from people who have tired they have ruined them. Look at linus tech tips... I would be mortified.
Just dont think its worth it. Its a beautiful machine... but for work does it really matter... rather have a smooth powerful experience and get work done faster, its not easy to find on the mac anymore.
Really depends on your use.