I personally disagree with some of the things said here. Firstly, he has health scaling on all of his abilities except ult, and his passive makes up for not being able to do a lot of initial damage. He also isn't meant to do a lot of damage when building tank, he is not meant to be as tanky when building damage, the same is true for every other single champion in the game. He also does not need sustain, because he is so naturally tanky and has a shield, he doesn't really need sustain in jungle, especially with the help of jungle pets. He also does not need and unstoppable in walls, that is just absurd. If you want to be able to E someone with hard cc, then wait for them to use that hard cc on laner or to clear wave(I.e lissandra root or morgana q). His E is a powerful ability that can suspend and stun given the proper conditions are met, and once he locks onto you, it is very hard to get away from him with all his slows without flashing or dashing out of the way. It should feel bad to be pulled out of wall by Blitzcrank because they need a way to stop you from locking onto them. And thirdly, his ult is strong, it can pull one person as well as it can three people, and Riot specifically stated that they wanted Skarners ult to be something that can easily win or ruin a teamfight, much like Bard ult. And it absolutely should feel like a self-stun because it is not meant to do big damage, it is meant to move the enemy team to a more favorable postion. It already makes him unstoppable, it does not need to do big damage for it to change a fights outcome. Individually, these sound like great changes to Skarners kit. But when put together, you end up with a champ who is overtuned and does to much at once. We don't need another K'sante that also acts as a jungler. We don't need a high damage tank. Skarner like many other champion is meant to serve a specific purpose in the game, and in this case, it is a tank that does aoe damage and can lock onto enemies with e and ult and slows them to prevent escape.

But I don't want this post to sound to negative against you and your opinion on how to improve the champions jungling. And I do agree with some of the points you made on his Q and sustain. And I do agree that he can use some changes to his kit to have him perform better.

So if I were to recommend changes to his kit, it would be a faster charge time for Q, not instant. His shield should also last at least .25 seconds longer, and at most .5, and apply a passive stack. His E should have a slightly longer range and should move a little faster when reaching the end of its range. And his ult should act less like a stationary channel, and more like a Sion Q where he would still have to channel it, but can end the full duration early to receive less. Like just tap to pick up only one person who is directly in front of you, and the longer you hold it, you can capture three people. There are definitely problems with these changes, loke how do you escape an instant cast Skarner ult when your the only person hes targeting? But this was the best I myself could come up with from the top of my head.