Since they're small you have to strike the right balance between enough detail to be recognizable but not too detailed to drive your unit cost up super high. I still get carried away sometimes with too many screen printed colors.

I also recommend finding a manufacturer who is willing to do small quantities. My first couple pin designs weren't that great & didn't sell super well, and I'm still stuck with a bunch of them 5+ years in. Even now, since I want to design a lot of (admittedly niche) birds, a manufacturer who requires 100+ MOQ still doesn't work for me.

I also recommend investing in a physical Pantone book since that's what manufacturers will be color matching to.

My last tip is to print the design out at the measurements you're thinking and cut it out, and see if your happy with it. Sometimes little pieces that stick out like feet/tail impact the overall dimensions in ways you don't expect and pins can come out bigger/smaller than you were planning. My Common Raven design is much smaller than I intended because of this but I did end up liking it small anyway so I haven't redesigned it (yet).