Here is an mit mod with some nice looking turrets you could use to decouple from space age while waiting for more custom graphics.

https://mods.factorio.com/mod/naval-turrets

Have you also asked in the factorio discord modding dev graphics channel? Lots of kitbashers in there

I’d rather not make my mod depend on an unrelated one just to get graphics, as it would bring us back to the same problem as the space age dependency. The MIT license on that mod only clearly applies to the code, and the author didn’t specify whether the art is covered, so I can’t assume I’m allowed to reuse the sprites.

I’ve requested new sprites on the forum and am waiting for replies. I’ve tried replacing the graphics several times before, but the results never matched well or I couldn’t get permission from the original authors. I know the dependency is frustrating, and I’m doing my best with the limited time I have to work on this project.

You are OK. I’m playing with space age anyway, but wanted to contribute to your graphics search. :)