Our changes slotted for 4.21 are aimed precisely at these.
Hyper carry: They flatten scaling ratios/utility and weight them considerably towards the early game. Further, the changes strictly add power to her at all stages of the game (including the early game) with the E debuff. Again, these changes don't reverse her power curve, but they significantly flatten it.
Even with these changes, her power curve is still the inverse of what she has been for years. You really need to stop pretending that having a weaker early game and stronger late game is anything but a huge deviation from what Cass has always been.
Single target: Okay, 4.21 doesn't really address the single target concerns, so, maybe it's not aimed at all of your concerns. True.
Thanks for at least admitting you aren't doing anything to move her back into the AoE powerhouse she has always been until the rework.
Direct damage: The changes do, however, undeniably get at the direct damage concern. The changes lower Twin Fang's best case damage (late game ratio down) and add some indirect damage (poison amp). The indirect damage amp depends on hitting E, yes, but, at the end of the day, that's solely a buff to her indirect damage.
When people say direct vs indirect damage, they don't really mean the poison debuff ticking away -- they mean the power of Q and W without landing E. Shifting power into the poison amp is not increasing the power of Q or W, it is just a different way of cramming power into E. I will give you that it does something for the theme of poison, but it does absolutely nothing to address the deleted gameplay Cass used to have when she had meaningful Q and W spells.
We're not done looking into Cassiopeia, and I know these changes won't entirely fix the problems you have, but I do want to point out that we share at least some of your concerns, and I think it's clear that we're in the process of addressing them.
You aren't addressing our concerns. Absolutely nothing has changed with the 4.21 changes. She is still the inverse of what she has been for years, she is the inverse of the reason I played Cass and spent my real world money on her. She is still single target, still late game instead of early game, still only has one meaningful spell outside of the ult, still only has one valid play pattern of go rambo all-in, and she most certainly hasn't regained any of her fun tempo from Deadly Cadence or feeling of a poisoner who is in control and wants to keep her distance until she manipulates the situation into her favor.
To me, her biggest fantasy/thematic elements translated into actual gameplay is the dichotomy of Cass' former life as a cunning/deceptive/manipulative courtesan and her new form of a monster that can't wait to strike hard and fast and RIGHT NOW. The play pattern of skirmishing with Q and W being powerful spells in their own right represented Cass' ironclad will holding back the inner monster to keep everything in control and manipulated into her favor as a remnant of her old life, and the "turn and burn" hitting R and spamming E represented her finally giving in to her new bestial nature. Both playstyles were satisfying and valid, but the truly satisfying plays were always the ones where both playstyles/natures were utilized.
Post-rework, she only has one play pattern -- that of a magic damage "autoattacker". Completely ignoring balancing, numbers, inverting everything about Cass' strengths and weaknesses, or fundamental reasons why Cass' kit is not ideal for a mage "autoattacker", this total loss of all choice in play pattern is why I absolutely disdain this rework.
Cassiopeia has always had two combat ranges (Q/W range and E/R range). Q/W are long range, and E/R are medium range-- I don't mean to be super controversial here, but E/R have never been 'Rambo' range. It's true that they put you at much more risk than the Q/W ranges, yes, and the update makes this her primary combat range, but 700 range is simply not Rambo range (is Ashe rambo range?), so I think that's a little unfair.
It is true that Cass' E is longer range than Ashe's autoattack. But Cass isn't going against Ashe in lane. She is going against other mages, or one of the many champs in league-of-dashes-and-burst-damage.
In recent versions, Cassiopeia functions most effectively at mid range, towing a very delicate line. Yes it's true that a mid range character is going to have lower range than most mages, since most mages are long range (or ther're assassins, in which case I'm not sure I'd classify them as mages in the first place), but that hardly makes them Rambo =/.
A great deal of common mages have equal or greater range than Cass' Q as it was. This made mid lane interesting for Q maxing because it basically came down to who lands skill shots more often, and dropping Q and W did not require/enable enemy to force an all-in engagement. Since she will always lose the poke war now with the meaningless damage Q and W now provide, her only choice is to close to E distance. But SURPRISE, every single mage in the world will be able to CC and splat Cass before she lands more than an E or two.
A similar scenario happens against assassins in that her Q and W used to give her an option in how to deal with these champs as they are long enough range that she can farm and poke without getting caught by a dash or CC. If you landed a few Qs and got a timely W off, you could whittle them down enough that you could risk an all-in. But now that she has no damage in Q or W (I have legitimately seen people stand in Miasma for the full duration and passive health regen + a last hit or two + lifesteal kept them full health), she can never whittle down the assassins enough to where she can risk an all-in. This problem is exacerbated since her initial release with the addition of so many champs with insane mobility.
Additionally, she previously felt good hitting a timely hourglass, because her poisons were still chunking the enemy for meaningful amounts. This let you outplay the Panth that would otherwise splat you because you could Q him as he approaches, drop a W on yourself, and hourglass as soon as his stun wears off. Then when hourglass comes off pop your ult and go ham with QW+E spam. If you timed and aimed everything right, you were rewarded for outsmarting your enemy. But now that all damage comes from E, it isn't a valid play pattern anymore to do this. Q and W don't chunk them enough while you are hourglassing, so you are now the one getting splatted even in your best case play.