I'm thinking I'd like to learn array languages (APL, J) and maybe use them professionally. Maybe their time has come.

Probably, especially given the boom of GPU/Tensor computing.

You might find Stefan Kruger's book useful: https://xpqz.github.io/learnapl/intro.html or his write up of the APL Cultivations (https://xpqz.github.io/cultivations/Intro.html)

Not sure where best to start with J, although finding it interesting reading through the Dictionary (https://www.jsoftware.com/help/dictionary/contents.htm) and seeing how it compares to APL