Need Rengar Help

I'm really comfortable on 6-8 junglers which I would confidently take into ranked, all AD assassins or tanks but Rengar is someone I just cant get right and would like to learn

1. I feel like his first clear sustain is so bad with only W giving some HP back. Even with Red-wolves-blue clear I use up my pot charges and smite and forced to gank now at level 3, or clear another 2 camps then need to go back because I'm so low. A lot of other junglers I can clear all the camps and still have HP to go gank or invade(Khaz, Graves, Xin, WW, Kayn, Olaf, any tank), but by not being able to do so I feel like I'm already behind. Is this something I just need to accept?

2. When do I skip jg item to rush Duskblade or Tiamat vs finishing the jg item?

3. When to go for bruiser vs assassin builds? If I'm ahead then assassin or more bruiser if I'm behind? Or dependent on team comps? Most of the time I'd feel like assassin is more beneficial to blow up carries, even if behind

4. Secondary runes have seen Precision with Legend: Alacrity + Triumph, or Triumph + Coup de Grace Sorcery Celerity + Waterwalking, or Ultimate Hat + Transcendence Which and why? Even at the pro level there's no consistency with secondaries