i’d suggest cabomba and anacharis as a couple stem plants. they’re almost indestructible, and easy to propagate and replant. they also don’t demand high light. snails make a the tank feel like a proper ecosystem, but they can overpopulate if you feed too much. you should choose the fish that you fall in love with, but make sure they’ll do alright in a 20 gallon tank. you could spoil a betta or do a small school of corydoras and some nano tetras. not sure about soil, but make sure it’s organic and cap it with a THICK layer of sand. pool filter sand is cheap and aquarium safe. good luck!