Oro's Sacrifice flicker Sion build - newbie need some help

Hello guys,

at first, I am new POE player, I'm currently playin for ~200 hours and I must said, that I totally love this game. I found a lots of friendly people in forum and also in game chat, I got a lots of tips, tricks and tutorials, but now, I need something like "build check".

I chose Oro's Sacrifice flicker Sion build as my first, but I think that I made some mistake or fail in my build. I doing maps, but sometimes, I getting IKs in T10 maps :D and sometimes, I am able to complete rare T15. I also think, that my DMG (especially to single target) should be better, but I don't know, how to improve my build :(

Currently I am using this items:

* for single target (boss) I am swapping my awakened melee splash support gem to Ruthless Support gem.

And this skill tree (jewels should be included): https://pastebin.com/txWWL6MZ

Can you guys please check my build and tell me, If there is any epic fail anywhere? My own plans for improving my build are:

1) try to swap The Wise OAK flask with Lion's Roar flask
2) buy 2 rings with chaos resists and use catalysts to boost my chaos resist rings to get chaos res (currently I have -60%)
3) buy stygian wise (hp/res/ele or fire damage) and jewell to it
4) pick all HP nods in passive tree (which are near Constitution node) for more HP

But I don't know, if these plans are garbage, or if it can really help me to improve this build. As I said, I am relatively new player and the game is still too difficult for me and I am still learning a lots of things.

Thank you for any tips, tricks, advices, whatever :-)

Have a nice day!