Giving APL a quick glance I find the topic at least intriguing.

So if I understand this right K is one of the more popular/recommended APL-derivatives nowadays. K is proprietary but there are a bunch of compatible implementations which are themselves open source. Is that correct?

Which APL derivatives (with an active/healthy userbase) besides k would you recommend to take a closer look at?

https://www.jsoftware.com/ - J has a pretty active community, or at least used to.

I’m an array language novice but my favorite one so far is BQN. Much better documented than K derivatives. Not sure you could characterize any array language as having a “healthy userbase” though.