I also agree that sion is nearly perfect, but some of the points you listed are in accurate
His W functionality is the same as before. With the old sion, if you destroyed his shield, then it would not explode. The reason it seemed like Sion was always able to trigger the second effect almost always before was because his shield would reach an insane value with his most popular style of play, AP sion. This gave the sion player alot of room trigger the W after taking some damage. With the current sion, the shield value is pretty substantial (compared to other similar shields) however much smaller than before, giving the enemy to burst through it before sion can trigger it. However, it can be argued that this plays into the skill of sion
His HP scaling is an issue, as he becomes extremely tanky quickly when playing jungle, however, becoming as tanky requires alot more farm if you go top. In addition to this, his hp scaling further makes going top less attractive, because he either ends up really squish if you had a bad time, or slightly more tanky if you had a really dominant laning phase. I agree that if I at least do well in lane, I should be un-killable, however currently this is not the case. In both jungle and top lane, I feel sion needs more tankiness to reward him for playing well.
E: Eh, it is an alright skill, it could be much better, giving the unreliability of sion, but I digress
R: I like the ultimate where it is, as it gives me global presence as a top laner. By taking teleport, I can base, choose a lane, ult that lane, get or kill or burn flash, which then my jungler can come and pressure, and then teleport top. Some of the issues that I do not understand about the ultimate is that it further emphasizes mobility meta. Sion's ultimate is perfect against champions with no dashes, but against a vayne or lucian, if they are somewhat competent, they can dodge your ultimate easily. This leads to another case of hey look mobility counters this, while non-mobile champions are basically unable to retaliate outside of flash.
Some overall points
Sion basically functions as a champion without an ultimate in the top lane. If not given the charge up time, sion's ultimate does not do enough damage or cc for 1 v 1 duels. This makes him a very weak dueling top laner (since he lacks the tools shen has, who has another non-dueling ultimate). However, I will agree that I am able to outplay my opponents from time to time with sion, and infact, I am able to win lanes that some other popular top laners struggle with, because sion has good wave clearing and situational skills