FYI, my KZ bluetooth cable came today.
The cable itself is identical to the stock cable that comes with the ZS3 - thin, black, a little stiff, with the identical ends/memory wire/memory wire sheathing. Since I use the silver upgrade wire on my ZS3 and ZST, I can't comment as to the longevity of the ZS3 stock wire.
But I did determine that the ZS3/ZS5 version also fits the ZST. It's only using friction of the rounded plug key plus the 0.75mm pins to hold it in, so it's not as rock solid as it would be on the ZS3 (where there's an incredible friction fit between the outer portion of the jack and the ZS3 shell). But it's definitely not so loose that it just falls apart. The force it takes to remove is somewhere in between the two, as evidenced by mowing the lawn followed by 6 miles of in-line skating, where they haven't loosened in any way. I was also worried about the fact that the ZS3 2-pin plug is straight (compared to the 90-degree 2-pin plug found on the ZST), but they fit around my ears perfectly fine.
So from my experience so far, I wouldn't worry about using any of the ZS3 cables on the ZST - stock, silver upgrade, or bluetooth.
The sound has the typical bluetooth sound - it definitely sounds a little compressed. But just fine for the typical things you'd use bluetooth for - yardwork, exercise, working around the house, etc. The ZS3 doesn't sound all that great with the bluetooth cable. I'm not sure if it's an impedance thing or the sound difference with bluetooth itself, but the ZS3 sounds noticeably "lower-fi" with the bluetooth cable vs the hard wired cable. I don't hear as much of a degradation with the ZST, so for now I plan to just use the bluetooth cable with the ZST.
One thing I don't like is that the control box is offset to one side vs being centered in the cable. It gives this weird "unbalanced" feeling when wearing it. It also makes wearing the cable behind your head almost impossible, because the control box hangs on 1 side, won't stay behind my head because of this, and the imbalance constantly tugs on the left IEM (loosening the seal). It's really annoying, so I can only wear the cable in the front.
I experienced 2 minor bluetooth skips/cutouts when mowing the lawn, despite my phone being 2 feet away in my pocket. It could have been 2.4G wifi or microwave interference or even EMI from the battery-powered mower I was using. But I have never experienced this with my other bluetooth headphones.
I'm still testing the battery life, so I don't know if it's the old one or the new one (I'll update this post with the results of the battery testing). Just like other recent HFers, mine also says Bluetooth v4.1 on the box but v4.2 in the instruction booklet. So I'll have to just use the battery life as the method to figure out which version I have.
I also haven't had a chance to see if there's any audio lag/delay, so I don't know if it's viable for games or movies yet. I'll update this post once I've tested this.
So far, it seems worth the $10 I spent on it.
Update: Over (2) full battery charges I get an average of 3.5hrs per charge (at 50% volume). Based on this, I'm assuming I received the latest version. I also removed the memory wire, which improved fitment and comfort considerably. Finally, I tested with some YouTube videos, and there's a very minor audio lag. It's not as bad as an English-dubbed kung fu movie, but you can tell there's a fraction of a second's worth of lag between a person's mouth moving and the audio. Based on that, I don't plan on using this particular bluetooth cable for movies. Finally, the cable does seem to be susceptible to some EMI sources. For example, when I leave my car and lock the doors using my keychain remote, there is a quick skip (interruption) in the audio. I haven't used it around the microwave oven, but that significantly degrades every bluetooth device I own. So I don't expect the KZ cable to be any different.
Since it appears that I received the latest version (based on the battery life and fairly stable connection), I'll share the seller where I purchased my cable from:
https://www.aliexpress.com/item/KZ-...grade-Module-for-ZST-ZS3-ZS5/32815821305.html