Thanks, man. However, I do not know the iMac so well. I know Macs since I have been using them since 1986 and I have an Electronic Engineering degree but I owned only an iMac, the G4 (Luxo). I have been relying on Mac Pros all the time: from the Mac IIcx to the Mac Pro 5,1, passing through Quadras, PowerMacs, G4s and G5s and a few notebooks.
Having said that, I believe I am in a similar situation (if I got yours): my old Mac Pro works still pretty fine albeit I would have changed it for something faster if there had been an option. Apple used to sell the Pro desktop for 2-3k $ and now has stepped onto the 5-6k minimum, which is far too much for me.
Like you, I feel I can easily wait a bit more, given the present options. That is why I would suggest you to wait at least the 2020 range before taking a decision.
The 2019 iMac is a beautiful machine, but very old in so many ways (BT, WiFi, bezel, thermal capacity, GPU, speakers, even the screen...). Perfect for any consumer, good for many prosumers, not very good when you move to users who are looking for a higher-end Mac. The iMP is outdated and with the Mac Pro out, it has become useful in very few cases, mainly for its price (and it is based on the same hw as the iMac, as far as Wifi, BT and webcam are concerned).
Given the fact that the iMac is not upgradable, I would go for it only when it reflects at least 90% of my requirements. I need BT 5.0, a faster WiFi (can't wire it) a good GPU and I don't mind a better webcam (I do a lot of video conferencing), HDR + Dolby Atmos for my relaxing time and a more modern design with smaller bezels.
I will retain the screen and use it as a second screen but I do not own an eGPU. Given you use the right software, you could use it as additional processing power as you said. In alternative - should your sw not support GPGPUs - you can still connect the second screen to the eGPU and lighten the burden to the internal GPU.
It is hard to know how long will it take to Apple to move to ARM but for prosumers/pro users I believe the transition will be slow. Therefore, the only real "big" step at the horizon as much as CPUs are concerned is the 11th Gen from Intel, finally on 10nm (Rocket Lake). It will bring TDPs finally back to "Apple levels" thus allowing for more tangible performance gains on Macs. However, it will very likely happen not earlier than the end of 2021, beginning of 2022.
On the GPU side, AMD should come out with the real RDNA architecture (the 5000 series uses the old instruction set), but given the Apple product line, you will never see a high-end card on an iMac.
As I said, there is a chance that Apple cancels the iMP and expands the iMac range to cover for its performance and price, if they incorporate some of the design/cooling features.
Personally, I am ready to wait up to another 10 months (till the beginning of November 2020) to see what happens.
They have already in their hands the CPUs and GPUs for the new iMP. Given it is not supposed to cannibalize the Mac Pro, it makes no sense that they didn't present at least a refresh yet. I guess they are either waiting for the iMac to be ready and present both at the same time (that would be quite strange) with a new design or they are aware that the space between the high-end iMac and the Mac Pro is quite squeezed and may get rid of the machine.
As for the iMac: the CPUs are very likely going to be unveiled at CES tomorrow and be available in volume by the end of February. Allowing for a 12 month March release or an 18month window in October/November. There are so many new technologies missing in the present iMac that makes it too old for the market that I am confident we will see it being upgraded this year. (T2/T3, TouchID/FaceID, SSD as standard, HDR, Dolby Atmos, Wifi6, BT5...). GPUs? Well, it all depends on what Apple wants to do and how much cooling capacity will allow for the new Mac, but many are already available and Apple surely has other SKUs of 5300/5500/5800? in their hands.
My opinion so far is that we could see the following scenarios:
* new iMacs (perhaps in Q2/Q3) with improved cooling than the iMP, covering from about 1.5 to 5k $ in two versions (21.5/27 or maybe 24/30). No new iMP. Perhaps a Space Grey version only for the high-end configuration? (remember the black Macbook?) GPUs could go beyond the 5700XT.
* new iMacs (Q2/Q3) with new design and GPU limited up to the 5600/5700 (2-4k $) and iMP (Q3/Q4) upgraded to the new design but with additional cooling (perhaps solutions like the back of the XDR panel) with new CPUs (Cascade Lake W) and GPUs (5700/5700XT and perhaps a 5800/5900) (for the range 4k to 8k $)
* new iMP (Q2) refreshed with new CPUs (Cascade Lake W) and GPUs (5600/5700XT) (4-8k). iMacs only in Q3/Q4 with new design, CPUs and mid-range RDNA2 cards.
I highly doubt Apple would switch to AMD CPUs. As fantastic it would be, it doesn't make much marketing sense unless it would be only for the iMac and mac mini with Ryzen. Intel still leads on the mobile range and it would be pretty absurd to "downgrade" the Xeon to the Threadripper, as much as it would increase performance in many SKUs and retain ECC, while EPYC CPUs are much less responsive for the daily tasks (and OS GUI).
They will just go ARM when they are ready, I think.
Perhaps the three scenarios are all a bit permissive as far as the TDP, but allow me to hope for the best!
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