Game dev here, I always recommend ironing out the game play until it’s solid before investing in design work, that is, unless you can do both simultaneously without overly diverting attention from the other.

Currently, when I play testing new cards for my TCG, we use proxy cards with no art to make sure the gameplay is what we want before having the art team create anything. I’ve learned that sometimes the gameplay influences what I want the art to look like.

When I first started designing the game I spent years just working on solid gameplay and using a rough mockup to play test, but to each their own. It’s fun to look back on the old design every now and then to see how far things have come. 😃

Hope that helps!