{EDIT}  So when I first posted the question, seemed like a pretty straight forward question, but turned out to be a bit of a complicated question..... 
> RAM: definitely need more than 16GB for 4K editing.  For shorter clips 32GB is fine, for some of the longer clips I'll be doing, 64GB should work, but I could see the potential to utilize 128GB as well.... 
> CPU: jumping from a 6 core / 12 thread 3.2 GHz i7-8700 to an 8 core / 16 thread 3.6GHz i9-9900K CPU definitely had a larger than expected impact, with about a 25% reduction in export speed on my test video.  I think mostly due to the ease at which the new CPU goes into higher core speeds.  My 8700 never got much higher than 3.8GHz boost, but the 9900K hit 4.9GHz on all 8 cores while exporting my test video.  It did not stay there, instead averaging about 4.4Ghz throughout the export.
> GPU: if you have a GPU that's not already on the list, you'll see benefits from moving to a supported GPU.  If like me, you're comfortable beta testing software / hardware, you may see added benefit from applying for the beta testing team.  My 1660 Ti showed up to 28% reduction in export speed vs just the integrated GPU, and up to 35% reduction in export speed vs just the CPU alone.   Of particular interest is the fact that it showed the highest reduction in export speeds when paired with the 9900K processor, while running just an 18% GPU load during export.
 
I think for the price, the CPU was an incredible upgrade for editing purposes.  But considering that an RTX 3070 gives about double the performance of a 1660 Ti in most usage cases, even at twice what I paid for the CPU, a 3070 now looks like it could potentially be the better upgrade now that I know I can add it into Premiere Elements 2021.{/EDIT}
 
 
4K TV's & monitors were barely a couple months on the market when I sold my last editing rig, 3930K, 16GB RAM, 660Ti GPU, 256GB 840 Pro SSD, Premiere Elements 8.   So I've kind of been in the dark on the new software / hardware for 4K video editing.   
 
Just bought a new camera with 4K 104Mbps recording capability.  I've been known to shoot up to an hour of video at family gatherings & get togethers which I then edit and distribute.
 
I know Premiere Pro was starting to use the GPU for editing tasks back then, which leads me to think a new GPU might be the best route.
 
Current rig can handle 4K editing, just looking to reduce time spent editing and encoding.  My current build:
 
Intel i7-8700 (non-K) CPU
Gigabyte 1660Ti GPU
EVGA Z370 Classified K motherboard 
Corsair 16GB DDR4-2666 RAM
OS drive: Samsung 970 EVO Plus 1TB NVMe SSD
Data drive: Intel 660p 1TB NVMe SSD
Scratch drive: Samsung 970 Pro 512GB NVMe SSD on PCIe reader
Backup Drive: Western Digital 2TB RED NAS drive
EVGA SuperNOVA 850 P2 Power Supply
 
Question specifically for this post:  Which would give more improved editing / encoding performance - i.e. best bang for the buck:
Upgrade the i7-8700 to an i9-9900K?
Upgrade the 1660Ti to a 3070?
 
I will be using Adobe Premiere Elements 2020 for editing / encoding tasks. EDIT: Premiere Elements 2021 was available when I went to purchase, so using PRE-2021 because it has added LIMITED GPU acceleration support.
 
Also looking to replace the 2TB WD with a WD Gold 4TB or 6TB HDD, and further down the line add a 4 bay external NAS.  Gold series for reliability and the large cache.

Thanks!
 
I'm checking into specs on Premiere Elements 2021 right now.  I wasn't even looking to get another camera till sometime next year, so I hadn't even started to do any research.   My initial though was the extra core speed and 16 vs 12 threads would give the 9900K a huge advantage, not to mention OC potential.  Then I remember Adobe talking about potentially adding GPU acceleration to Elements back when Elements 9 came out.   
 
Fortunately Olympus had a flash sale on while I actually have some extra cash.  BTW, If anyone is curious, the camera I picked up is the Olympus OMD E-M10 Mark III (scheduled to deliver tomorrow) which is on sale with both a very basic 14-42mm lens, and a pretty decent fixed length 45mm f1.8 lens.  Currently on sale for $589 after promo code OMDBUNDLE through Midnight tonight.  Even though it's almost 4 years old already: between the camera being a better version of the "Pen" model, and the 45mm F1.8 lens normally being $399 by itself, I couldn't pass it up :)  
 
Guess it's time to put the research cap on & dig in, starting with some hands on with the trial version of Premiere Elements 2021 !!!

That's an asinine list of supported GPUs. Why can't it support a full generation of GPUs instead of just specific models?

Based on some very basic 4K editing and encoding of a 28 second long test video from a 92.4MB original clip shot at 26Mbps (not sure yet why camera wasn't shooting at full 102Mbps bit rate), I think I'll be starting off with an upgrade of RAM to 64GB.  Test encoding used on average 13GB of my 16GB system RAM just doing a time stretch of the clip to 56 seconds.  
 
I'll wait till a good deal pops up on the 9900K (thinking maybe Black Friday), and later on a 3070, even if support isn't added for it in Premiere Elements.

From what little I've done so far, the 64GB RAM is a definite improvement, with RAM usage exceeding 24GB a couple times for 3-4 minute clips.  Just purchased the i9-9900K, should be here next week.  I know it won't be a huge improvement, but when I do get to much longer projects, a 10-15% improvement (before overclocking) is nothing to sneeze at  and for longer project's I will definitely overclock the processor now that I'm going from a locked to an unlocked processor!
 
I signed up to be in the Beta testing group for GPU acceleration in Premiere Elements 2021, but so far, no word back.... so I tried Ranmacanada's suggestion / link, at first glance appears to be no luck, there's no "cuda_supported_cards.txt" file... HOWEVER, when I ran GPUsniffer.exe, result was different from the link & instructions: 
c:\Program Files\Adobe\Adobe Premiere Elements 2021>gpusniffer.exe
.
.
.
CUDA system device count: 1
Found CUDA device index: 0 Name: GeForce GTX 1660 Ti
No capability restrictions detected - adding device to list
OpenCL system device count: 1
Found OpenCL device Name: GeForce GTX 1660 Ti supportsSSG: 0
Skipping nVidia OpenCL device
Finished gpu initialization in 0 ms
 
Kind of looks like it automatically added the GPU to Premiere Elements.  Probably won't get to try it out till after the CPU comes in though.

FitzFox
 
I've never seen Premiere use CUDA/gpu properly. Tried on 2080Ti, RTX Titan, Titan Maxwell, 3090, even back on 680s.. In heavy editing it makes little difference outside of certain canned effects. I'd definitely go with CPU power above all else. Puget systems have some good benchmarks on that.

Premiere Pro, or Premiere Elements??? Elements has never supported GPU acceleration (from what I can tell) until the current 2021 version...  Back when I had my GTX 660 TI, Premiere Pro CS6 (trial edition) definitely used the GPU for editing and previews.  Puget also showed that CS6 did utilize the GPU https://www.pugetsystems.com/labs/articles/Adobe-Premiere-Pro-CS6-GPU-Acceleration-162/#Results turning a 5+ hour render for CPU alone, into a 1 hour render with a 660 Ti card.  Of course, this was about the same time I had to stop editing video and sell off my 3930K based editing rig :(  
 
Got off work early tonight, so checked out Premiere Elements after the GPUsniffer thingy... my 1660 Ti still shows unsupported.  Guess I'll just have to hope Adobe adds support for more cards somewhere down the line, & just be happy with the little boost from the 9900K in the meantime!

A 32 core 3970x Threadripper or a 16-core 5950x. More video ram the better. At least 4TB of SSD space on M.2.

Managed to get off work a few hours early, and this was on my doorstep:

(in original retail packaging of course) 
 
So, I'm about to do some testing with & without GPU acceleration with the 8700, and then with the 9900K!