The reason it's so hard to shave that coat is because it's a double coat and because the hair was dirty with oil(naturally oily like healthy hair, not like motor oil).
If you shave him again, try bathing him and brushing and make sure he's thoroughly dry, then shave him. The clippers will go through the coat easier if it's clean and dry. Shaving against the "grain" is fine with most blades, I often use a #7F in 'reverse' on that type of coat with really good results.
Since you've bathed him now, you might try to run the clippers over him again when you go to "touch up" the spots. I bet the tracks and unevenness will disappear almost completely.
The color difference is common, since the undercoat is often a different color than the harsher outer coat hairs. The softness is also due to the undercoat being soft. He won't be as waterproof as before and the hair may get dirty easier for that reason. But since he's easier to wash, it's not much of an issue.
Nice job. I've seen groomer trainees that did way worse jobs than that. (And in case people fuss at you about not ever shaving double coated dogs, it should grow back in just fine, I do tons of double coated dogs and the coats always grow right back in fine).
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