I suspect we'll see Cackle remain as sustaining a single effect as a default, though I wouldn't be surprised if there was a feat or ability to alter this as suggested, if only for your hexes. This upgrade is quite potent (getting extra Sustains is currently rated as a 16th level feat), but could be an interesting niche for Witches in general as the sustain caster.
They already got an ability in this niche as the 20th level feat Echoing Cackle, which automatically sustained your hexes for an entire minute with a single Cackle, but that doesn't necessarily preclude them exploring the space earlier. I think that 10th level would be about fair to start getting multi-sustain, albeit just limited to hexes. It would certainly enable them to push hexes a little harder, and explore cantrip hexes as well, since you couldn't really multiclass to abuse this.
What I really want to see, though, is the option to add riders to Cackle. A feat that enables you to Demoralise as a free action following Cackle would be highly thematic and quite potent.
I like both of these ideas! I think 10th level is bit late, I'd probably make it 6th or 8th. I definitely like the idea of adding debuffs to Cackle itself, too.