Witch Occultist CI Cold Snap

Hi there! So I decided to try and make a new build I've never tried before. CI Cold Snap Occultist. I have never played CI before, actually I've never played anything other than different variations of Cyclone Duelist (2h sword, dual-wield, facebreaker, etc). My idea is very simple - shatter an entire pack of enemies with just a single cast of Cold Snap while standing safely a screen away from them.
Rn I'm level 72, just finished the story and did some T1 maps. and the build works as intended. I have 6k ES (not much I know but I just crafted this gear to switch to CI and start doing maps, I hope it will improve soon). BUT. Here are my concerns:
1. The build is probably going to fall off pretty hard once I start running higher level maps. I've already picked up most of the offensive passives I wanted. Now it's time to grab some defensive nodes like more ES and spell block while holding a shield. What do I do to be able to one-shot enemies on higher tier maps?
2. I feel like my biggest problem rn is lack of dex. I can't level up my cold gems like hypothermia or Herald of Ice because of it, even tho I already picked up 2 +30dex and 1 +20dex nodes. My gear is pretty bad RN, outside of 6L 600ES Vaal Regalia which i bought for 2ex, everything else is self-found or self-crafted. I just simply used alt orbs until I got some ES, then Regal and master craft resistances which are barely capped. So I can't get dex on my gear atm. So I was thinking of anointing Utmost Swiftness for now, until I get better gear, is that a good idea?
3. Since this is my first time making a build at the top part of the skill tree, I feel like my points are not distributed as efficiently as they could be. I tried to compare it to some other cold builds but they are going for more dot damage, and I'm going for one-shot so it's not very accurate. Do you see any places on my tree where I could save up some points? Maybe choosing a different route somewhere? I have many regret orbs to spare so I don't care, I can respec the whole tree just to save up 1 or 2 ppoints.
4. I'm using Winter's Bounty unique jewel to get power charges instead of frenzy. And I'm gonna pick up Forbidden power as my last ascendency passive. Do you think it is worth it to spend 4 passive points to grab +2 max charges so I have 6 at the end? And is it worth goin for Discipline of the Forbidden to get more out of these charges?
5. Mana problems. For now I'm using clarity, it's still not enough but better than nothing. Is Mystic Bulwark do the trick once I get more chance to block spell? I hate wasting passives for mana regen, and I hate using mana flasks.
6. Last thing, are there any uniques I don't know or haven't thought about that could help me improve this build further?
This is my current POB: https://pastebin.com/P0azuA4c (I hope it works, I've never used this before)
I want to spend the remaining points on chance to spell block and ES wheel above Scion.