I agree with just about everything you've said. However, SLKR's survivability goes through the roof once he gets his ultimate off and, more often than not, he will be in his ultimate for all but the smallest of windows once he gets to it the first time.
I think a small tweak here is what I would like to see. If the only thing that changed was a slight decrease in the rate of ultimate gain after the first use of it (even 10-20% slower) I would be happy. The fact that he can build towards his ultimate at all while actively in his ult stance seems silly to me. GL or not, that seems a bit much imo. But, again, even this slight adjustment could be the nudge that makes sense to me.