[RodgerTheGreat]

ngn/k is a performant FOSS implementation of k6, but it has many constraints and limitations that may make composing large programs difficult. It may be the best dialect for tinkering and learning the language. https://codeberg.org/ngn/k

oK is a slow browser-based FOSS implementation of k6, with fun bells and whistles like "iKe", a visual livecoding environment. ngn/k and oK have diverged a bit, but are largely similar: https://github.com/JohnEarnest/ok

Q is a K-derived language built into kdb+; this is probably the most commercially viable dialect to learn, with the best educational materials (I recommend Nick Psaris' Q Tips), but it is proprietary, with (last I checked) free licenses available for personal use.

If you've seen Decker you may also have been exposed to Lil, which is similar in many ways to Q. It is more verbose and less expressive than Q or K, but I've generally heard people say they find it comparatively accessible: http://beyondloom.com/tools/trylil.html