That is my biggest fear. When we come back from vacation, we spend days processing photos. On my old Mac Pro w/ High Sierra, our edit session gradually lags behind as we go from photo to photo until it crashes. If we re-launch Photos once in a while, or even reboot the Mac, that helps. We are not pros but serious amateur photographers. At times, we do like live photos for long exposures for things like waterfalls/streams when we don't have a tripod. If Photos become totally intolerable, I probably have to use Photos for changing the Live photos / Portrait or do it on the iPhone/iPad and then import it to LR. We also have short video clips mixed in also. Without those formats, I have a relatively fast Laptop to use LR. The problem is all the information we tag and create albums is lost or has to be redone. I am also not sure my wife likes to lose the ease of use of Photos than LR. LR is not a problem for me. Sad to say, Apple has not been good in resolving bugs on the Mac SW like they have in the past. These crashes has been there for years.
For others, I really like the iMac, but there are some advantages with the portability of the MBP. Maybe when my old iMac dies, used for general computing and external monitor, we'll upgrade it.
As others have noted, you seem to have a software problem as much as a hardware problem. And the more you commit to Photos, the worse it can get. It can be difficult to transition out, especially if you have no choice. So easier to make a gradual transition. Editing Live Photos in macOS was only thing you cited that Photos could do that other applications can't do, and even with that you could use your iPhone instead probably as well, as you note (I'm like Mr Hewitt, I never use it so can't say for sure). Or use Photos JUST for that; I use Photos but only as a means of sharing a very very small subset of my bazillions of images, not managing a large library. It doesn't do that well, as you've discovered.
And note that Lr's camera app takes raw photos on the iPhone; AFAIK Apple's Camera.app still doesn't do that, and it's really nice to have.
And yes, you've got issues with having committed all that tag info to Photos; you might be stuck in Photos with your existing images for a while. But each new image you add makes that worse, not better. Most every other image organizing applications don't have that problem as most all I can think of can put the tags where they belong, in the image metadata.
As noted, the pure benchmark speed of the iMac 2019 is faster than the old MPs; don't know about the new ones, but even when the MPs were new it was the case that you had to buy a mid range one to get better Ps and Lr speeds than top spec'd non-MPs. Video would be different obviously. Tests here:
https://browser.geekbench.com/mac-benchmarks. And look on Puget Sound System's web site for tests on both GPUs and CPUs to see similar results, but with Ps or Lr tests.
And as to the Yuryev videos, I'd seen those, although I don't recall much on thermals. Note that "average" is average, and not "max." Not uncommon for those to throttle back on most machines, and rarely an issue for still photographers vs videos. But the last video he did is a real world comparison between the MBP 16 and iMP, and his conclusion is that the MBP fares very well against it considering it's portable ("the closest these machines have been ever"). I'd also recommend viewing it if you're considering an iMac.
And AppleInsider did some thermal testing, and found no throttling, that the 2.4 ran at a consistent 3.1-3.2 after lots of stress. And as the tester noted, it does jump up to 5 "right off the bat."
I would think it would be a dandy machine for photo work, thinking of replacing my laptop with one eventually (and before Apple decides to make another "improvement" like the butterfly keyboard :-D ).
