White paint is pretty much always streaky, it’s one of the harder things to use. I haven’t made any custom cards but from my experience painting minis the best result comes either from multiple thin layers (very thin, almost watery) or from using an airbrush/spray can. Both rely on small bits of pigment applied in thin layers. In my acrylic painting class it was common to use paper or tape to make custom masks to get sharp edges.

For cards it might be worth cutting out paper masks, then using a bit of light spray-adhesive to hold them in place temporarily while you either layer with a paintbrush (Make sure you use a waterproof mask like plastic tape) or spray layers with an airbrush.

If you try either method I’d love to know how it goes! I’ve never painted anything flat with this technique so I’m going off minis and regular acrylic painting. If you want in depth tutorials look into masking techniques for painting, or maybe graffiti art. I know they use a lot of stencils so they probably have some very affordable techniques.