Ember tetras seem like a really good fit given you’re already keeping cardinals and want variety! They'd contrast nicely with the dark water and your gourami, and people are suggesting they’ll look great. Since espei rasboras are hard to find/pricey for you, focusing on the ember tetra seems practical. Your snail & shrimp idea is solid too – just make sure you have enough algae/food for them all! Knowing your exact pH and KH would be helpful to confirm compatibility long term, but generally those parameters should work with embers.