Concrete for small castings

What would be the best concrete I can get at the home store to make castings like small pots, or larger 3x3x2” panels? , concrete art etc . I used regular concrete in the past with a melamine mold, and gave me smooth results but the aggregate is too big and over time the thin cream layer breaks away showing voids.

Do I use a mix with small pea gravel as the aggregate? Better to mix a custom blend?

Any tips will be appreciated!