Totally agree. There is no solid way to quantify how much to spend on a build for photography or videography. TR could stand in the gap there.
The problem is it's hard to establish a baseline load.
Came to say this, and basically everything you put after.
I'd think that any decent modern system today with a 4+ core CPU, 8 GB+ RAM, a SSD, and any discrete GPU will probably be 'overkill' for average DSLR/MILC users just processing RAWs, but probably inadequate for professional or enthusiast content creators. The hard part will be quantifying the level of performance and correlating it to specific hardware bottlenecks if possible.
That last line nails it.
I will share one bit of personal experience. I upgraded from an already excellent XPS13, call it the 2017 model, that has a 7500U which is a dual-core CPU. The laptop otherwise has an excellent screen, keyboard, touchpad, NVMe drive, and 16GB of RAM.
And I had three complaints with it. First two were that it wasn't a flipper (2-in-1), which is useful for retouching, that the screen while nice was a non-integerly scaled panel that required scaling to use and non-integer scaling is imperfect in Windows.
The third was that the 7500U was just plain slow when it came to doing multi-layer processing, especially stuff like HDR used to expand dynamic range*, and Intel had just released their quad-core ultrabook line. I replaced it with an 8550U ASUS 2-in-1 which otherwise has the same basic specs, but swaps SATA for NVMe, and is otherwise inferior in every way, just not enough to keep me from using it, or to downgrade (quite literally) to Dell's XPS13 2-in-1.
Going from two hyper-threaded cores to four makes a pretty big difference here.
Now, going from a quad-core hyper-threaded CPU in the i7-6700K in my desktop to the i7-8700K didn't make too much difference, but in both cases, this is workload dependent situationally dependent. On the desktop, I have four screens going and I don't mind waiting as much, and don't have to wait as much; on the laptop, I have one screen, and I'm waiting because there's a thermal budget involved.
And that's just CPUs. The issue with GPUs as noted for content creation is that their utility is very highly workload dependent and using modern content creation routines for GPU benchmarking is a good if complex issue. I'd like to see a 'content creation' build that includes baseline benchmarks that can then be updated both with new hardware and new software.
To add, a large part of the deficiency of GPUs when talking about utility for content creation is the lack of implementation of hardware acceleration in the software itself- meaning that benchmarks would likely need to evolve a bit quicker than they do on the gaming side of things.
*[when talking about HDR, there's slinging the sliders to the stops, and there's using multiple exposures to keep highlights from blowing and shadow detail from being lost to noise- the choice is personal, of course, but mine is to use the technology to produce cleaner final images]