Hey everyone, I figured - because I saw a handful of posts adjacent to this build/flicker/ephemeral - I'd share with you all my guide for this build.
My guide which goes in-depth and has a video guide attached has been available in the forums for a few weeks, but now figured I'd also share it here on Reddit.

This build is a ultra tanky all-rounder for all you Flicker Strike enjoyers, that offers a practical solution to the extremely annoying defensive issues other high-DPS Flicker Builds have (think Farrul's/Paradoxica).
It can run all map mods, albeit 2 mods (-block chance/charge removal) cripple it heavily. Without wisp crit scaling, tri added dmg / crit mult is runnable, and Simulacrum farming is a breeze.
I have been running Wave 30 Service with this with 5 Portals up 99% of the time.

It's not using groundbreaking new tech, but has been forgotten about in the last few leagues because it is pretty off-meta, but especially because it was heavily nerfed in 3.21.
The base concept has been around for a long time, I did not discover the potential of Ephemeral/Aegis myself, but I picked up "maintaining" a build guide for it to make the build easier accessible for everyone because I have been playing this a lot and have a lot of insight on gearing options when past options get dropped/nerfed, etc. because of it.

Links are at the bottom of the post.

Most of this is copied/paraphrased from my forum post, but here's a breakdown:

How does it work

Damage Scaling

• High Energy Shield adds max Lightning Dmg trough Ephemeral Edge.
• Resolute Technique Corruption Implicit makes all hits hit, never crit, solves Accuracy.
• Voice of the Storm makes non-crits Lucky, Precise Technique + CI makes them do 30% more dmg.
• Abyss Jewels for more added flat lightning.
• Intimidate, Bottled Faith, Charms, Mark Node and TWWT makes Enemies take increased dmg.
• Tons of Exposure and Penetration.
• Double Damage Chance from Lethal Pride

Summary: Lucky, Flat Added, Increased DMG on enemy, Double Dmg, Penetration

Defensive Scaling

• High Energy Shield (7k+)
• High Evasion converted to high Armour
• 50/50 Block
• Massive ES regain on blocked hits (~1200-1500 ES)
• High Leech + ~12% of ES on kill
• Immunity to Bleed/Corrupted Blood & Elemental Ailments & Chaos Dmg

Summary: ES Stacking, Aegis + Block, Overleech, Immunity to everything

Why would you want to build this / or not?

Pros

• Ultra Tanky
• Optimized to facetank, can turn off your brain
• Good DPS
• Deathless 30 Wave Simulacrum, etc. (if the mods aren't rippy you can facetank Krosis Slams)
• Won't die to almost any one-shot
• Base concept also works with other skills if you want to play some other melee skill instead

If you want to farm juicy wisp maps with it, get crit reduction from e.g. chest implicit or cramp in a armour mastery in the tree, blue wisps scale crit mult and that hurts a lot.

Cons

• Biggest weakness of this build is boss damage, some bosses take a few seconds to chew out if they are beefy, but all non-uber content is totally fine unless you try to run this for 10d in this economy
• Without Awakend Multistrike you can sometimes run out of Frenzy Charges when there's only 1 Enemy, pretty non-issue tho, and you can get it back with pressing your frenzy skill for ~1 second.

With proper investment you can kill Ubers with it, but it's not nice to do so at all, you die to some slams, uber shaper is annoying as hell, etc.
I did uber challenges with it, but expect to waste most of your portals if you're still leap slamming and expecting to facetank like you do everything else.
That being said, Flicker Strike bossing is generally a bad experience unless you build a glasscannon, especially if you don't have an instant movement skill other than leap slam socketed, so if that's your goal, don't build this.

Before this sparks confusion yet again, how does this sustain frenzy charges?

It relies entirely on frenzy charge on hit/kill chance. This works especially well since the base chance from flicker strike was buffed this league, no need for Farruls shenanigans as with glasscannon builds.

Math time!

The relevant calculation here is against a single unique enemy (map/wave boss), because 8% on kill + 20% on block + this calculation is enough to sustain against mob packs anyways.

• 25% from 20% quality Flicker Strike
• 10% from hits against marked enemy
• 8% from hits against unique enemy

That makes 43% chance to gain a frenzy charge on hitting a marked unique enemy.
You hit 3 times per expended frenzy with regular multistrike, 4 with awakened multistrike.
That means you have a 81.48% chance to gain a frenzy charge whenever you expend one (aka pressing flicker strike), or 89.444% with awakened multistrike.
This only accounts the probability to gain at least one frenzy charge. You will also be getting two or more per expended frenzy charge on occasion.

What if I do run out tho?

Press frenzy for about 1 second and you're at max charges again.
This is mostly relevant for bosses without mobs in a area before them, or when you run regular multistrike and just run out against a boss.

Links

PoB: https://pobb.in/PsGX1_UVzgJM
Forum Build Guide: https://www.pathofexile.com/forum/view-thread/3472898
Video Build Guide: https://youtu.be/IhkxJW2cI6s

Both of those showcases were done with a 40d budget early on in the league:
Mapping: https://youtu.be/zfK6mWJ0H9w
Simulacrum: https://youtu.be/jfwn3pbyFJA

Hope this helps anyone that doesn't scour the forums that either wanted to build this anyways or didn't know about this build.

EDIT: Formatting