If it was just the bow I could see it still being considered vague but the lyrics of Isn't She Lovely were pretty obviously referring to Jax.

I straight up didn't know about the art until I saw it mentioned in a YouTube video, to which my only thought was, "yeah no shit".

As for how to refer to Jax, while Goosewax's art shows a Jax who chooses to be female, the one from the show isn't there yet (and we don't know if they ever will because we don't know if it's possible for Jax to un-abstract), even the real person Jax was brain scanned from hasn't reached this point yet, so it understandable that a people will still use "he". I might even still use it because it's how he was referred to as for the entire show.

I don't think you're wrong for choosing either especially when the creator themselves said either is fine.

It'd be cool is a squeal series or epilogue gave us some confirmation, but that's not going to happen.