I've been PoBing this build for a couple of weeks now and I think I've got a pretty decently optimized league starter tree+gear. https://pastebin.com/KffuSD0r

4 million pinnacle DPS with easily obtainable rares and no PoB warrior BS. No flasks except rotgut, nothing ticked that shouldn't be, etc, no additional curse, no influenced gear except a life % hunter belt. Fork socketed into 6 link. Ballistas turned off, guard skill turned off. Far shot estimated at 40 projectile travel distance instead of 60 (at 60 DPS shoots up to 5.5 million)

With single target gem swap (fork for elemental penetration, hypothermia or inspiration - NOT barrage, because it makes each hit do less and thus freeze less) and ballistas turned on, DPS goes up to 6.7 million.

Able to freeze EVERYTHING except 70% damage reduction ubers. 17 million ailment threshold for minimum freeze while normal pinnacle boss ailment threshold is 6.5 million.

So it freezes pinnacle bosses for approx. 0.8 seconds with NO freeze duration gear. Can swap in Southbound gloves or Expedition's end for uber bosses if needed in the endgame, though by then I would imagine you'd deal so much more base hit damage that it might not even be needed. Freeze reduces pinnacle boss action speed BY 70%, making it much easier to dodge everything.

Ice shot is also a very decent mapping skill, with fork (or chain, I'm not sure which is better yet) it should have no issue mapping especially since everything is going to be permafrozen. When you transition into cluster jewels, the most important is actually the non-damaging ailment effect medium cluster that gives the Blast-Freeze freeze proliferation keystone, which will just make mapping even smoother. The same cluster jewel also gives Stormrider which enables power charge generation.

Downside: Relatively low life (4500 in current PoB) and defenses especially lacking in physical mitigation. I chose to go ghost dance just because I've never tried it before, but will probably end up swapping to hybrid armor/evasion gear, getting endurance charge when hit small cluster to get a little bit of physical mitigation, and swapping grace for determination, or getting more reservation efficiency and just running both.

HOWEVER, we are freezing and chilling everything all the time. Deadeye is also an extremely fast ascendancy with tailwind, meaning you are basically always moving and it's very easy to avoid damage without even trying simply by stutter stepping. Spell suppression is capped, we have 73% evade, and steelskin (or molten shell if you prefer an armour build) on left click, so it's not bad at all. I've played 'squishy' deadeye builds before and they always end up being way more survivable than PoB implies just because you have such an easy time dodging everything, even if you're not trying. Another thing is that once you get some additional arrows on bow/quiver you could probably even drop endless munitions and pick up wind ward, which is basically 30% less damage taken provided that you're only getting hit once every few seconds.

Might also have some mana problems if you miss shots. Mana leech is able to sustain the 3.76 attacks per second, but not if you miss half your attacks. Ideally there would be an upgrade into a -15 mana cost to socketed attacks/additional curse/crit chance influenced chest at some point.

For pantheons, I'm going to go Ryslatha for life flask charge generation since otherwise it might lack recovery. Life flask with instant recovery on low life+Corrupted blood immunity. And Arakali to make up for lack of chaos resistance.

At the very least, I'm sure that this will be solid at mapping and will have an easy time with permafrozen normal pinnacle bosses to get watchstones - I've done them with 1-2 million dps in ssf in 3.19, so with the 6+ million this build has with ballistas out and fork swapped out gem socketed, they should be a complete joke. For Ubers, very likely doable with more endgame gear, though as always with a squishy build it depends on your ability to dodge. I make no promises though until I actually do them myself.

Endgame transition

This build also has room to scale all the way to endgame. I stupidly didn't save it but I had an endgame gear setup with cluster jewels+timeless jewel+watcher's eye+forbidden jewels which I got up to 20+ million single target, 30k+ effective hp vs elemental damage and 20k+ vs physical with guard skill turned off, and permanent flasks from Nature's Boon. I'm sure someone who knows the game more than me could do way better too. There's nothing crazy really, you just transition to cluster jewels (I recommend bow damage large, effect of non damaging ailments medium, armour small for endurance charges), upgrade your jewels and get better items and your damage and survivability goes way up. I'll probably make an endgame PoB later and link it here when I do.

The 'Fury of Nature' hidden forbidden jewel ascendancy notable seems absurd for this build too - 100% more effect of non-damaging ailments for 2 jewel slots.

With the new Lioneye's glare, I also think elementalist might be worth considering endgame if you can manage to find one somehow. It gives you far shot which is worth an entire ascendancy point, then you could take the all damage shocks ascendancy node and go Omni, alongside 30% more damage from convergence, additional -25% ele resistances, and 50% more effect of freeze. But that's way out of league start territory and as far as I tell elementalist has no way to get additional projectiles without sacrificing a support gem. Without lionseye and omni there's no question that deadeye is better.