J mostly looks good but [) looks almost like a fancy capital D which can be distracting and =# run together at smaller font sizes which is a little weird. Those are the only things that jump out at me but I did not look hard, overall I would say it is one of the better fonts I have tried for J.

The big problem that I am having with this font is that its narrowness makes it difficult to find a fallback font for APL/BQN that plays well.

good idea. since i don't code in J, can you supply a 2-3 line quintessential J example to demonstrate this?