BY THE POWER OF GRAYSKULL, YOU SHALL HAVE YOUR ANSWER (mostly)
ZST is the most grandiose sounding of the bunch. Lots of energy, big bass, bright treble, aggressive midrange. ZS3 is imo warmer, darker, more natural sounding, and more mid-bass focused. Less fatiguing too. Soundstage is similar in size but ZST is a step up in terms of raw detail and accuracy, imaging quality, and instrument separation. Those are it's best qualities I feel.
ATR and ZS3 are pretty similar overall but with some key differences that I covered a while back. Post 1 / Post 2 Hopefully those help.
My ZS1 isn't the same as the model you can buy right now but I have spent some time with the current revision. It sounded identical to my ZN1 Mini so I'll be using that. ZS3's lower treble is more prominent while the ZN1 has better extension. They both have a lovely, smooth midrange though I'd give the ZS3 the edge. The ZN1 comes across a bit veiled in direct comparison. The ZN1 is a touch grainy top to bottom lacking the refinement of the ZS3. ZS3's bass is slightly less prominent and better controlled.I find the ZN1's bass to be a little hollow and heavy. Speaking of weight, the ZS3 has a thinner, more nimble and distant presentation. If you want a fun, bassy signature that doesn't sacrifice much in the way of treble/mid quality, the ZN1/ZS1 is a good pick. If you want something that's a bit more balanced, the ZS3 would be the better option.
I'm not going to compare to the ED9 since I have such a love/hate relationship with that earphone. At times it sounds amazing, at others the treble is too wild and splashy. I'll leave that to another.
The original ZS1 has a crossover covering the rear of the treble driver and a brass ring in the nozzle instead of the standard mesh you find on most earphones. You can sort of see the crossover in the image below. You can't buy this one anymore. Talking to KZ back in the day through their AliExpress store, it seems this was a limited release.
This thing is a super smooth, highly refined bass cannon. Much warmer and bassier than the current version imo. Personally I think the current model is technically the better of the two, but I like the original more. Way too much fun to listen to. The bass is unnecessary and it's awesome. Very unique.
Thank you HE-MAN!