J is really nice, though different than Q (more directly a dialect of APL than a derivative).

Though if you want an open source k, ngn/k was just released under the AGPL a few days ago.

I'd recommend starting with J, though, because there are a lot more resources for it, there's a mobile app for it (that's really nice), and it's the last thing Iverson worked on.

IIRC, Iverson commented somewhere that K's (Arthur's) practicality wins over APL/J's (Iverson's) purity in some places; specifically,

K/Q does scan/reduce from left to right, which is easier for most people to reason about and use, even though it makes "-/1 2 3 4 5" different from "1-2-3-4-5" and thus less "pure"/"consistent" in some sense.

J's trains and forks (and APL's simpler products) are much more confusing for people than K/Q's juxtaposition and projection.

Can't remember or find Iverson's comments now. He might have made more, or I might be misremembering/misattributing. But I agree with these and think K is a better start than J (though not sure between APL and K).