What I would avoid, Bill, is ever trying to identify "those huge parts of Photoshop [which] may apply to everyone else with different graphic processing goals". Photoshop has endured a decade-or-so of being labelled as for graphic artists, not photographers, and of course there is
some truth in that - until the moment that you have a problem and remember some weirdly-named tool....
My suggestion would be to focus on things you just can't quite get right in Lightroom, just like those I mentioned. Once you have an objective, learning how to achieve something is much easier and you'll notice interesting stuff on the way. I'd suggest watching as much of Julieanne Kost's videos as you can, and going back to them. They're sensible, not gee-whizz. Also, she tends to focus on new methods. That's important because one of Photoshop's problems is that there are 25 years' of techniques. Many of them are old and now only have very-specialist uses, yet how do you recognise those you should ignore? For example, I know of 20 ways to do B&W, but I would only recommend one (the B&W panel). Julieanne highlights new methods, and they are there because they are indeed better than those they replaced - usually in terms of flexibility, if not quality too.
Just don't be tempted to wall off any part of Photoshop! You never know when you'll have a lightbulb moment.
John
PS For an example, photographers quickly see the scope for doing cloning work on another layer in Photoshop, keeping the image untouched. But then there's something called the layer "blending mode" which may seem like one for the graphic artist. But a blending mode is just like adding pixels together in different ways, so you might choose one mode to affect image colour without darkening or lightening, or select another to make the picture darker without changing the underlying colour etc.
So I had a car rally photo taken in falling snow, but I didn't get enough snowflakes to convey the feel of the event. I could copy some of the snowflakes onto another layer, but they came with some darker background (imagine selecting only a snowflake in the wind) and it wasn't so simple. While I could change the size of the snowflake layer, twist it a bit, and maybe flip it horizontally and avoid making them loo obvious copies of the real ones, these background pixels meant I couldn't hide my handiwork. But one blending mode is called "Lighten" - only add those pixels that will lighten the image. So it made the layer add only my lighter new snowflakes to the picture, ignoring the darker background pixels. Now I could add
as many snowflakes as I wanted. The real achievement though was getting the photo in the first place - the rally was on a mountain top, windy, freezing, raining when it wasn't snowing, and dark!