In a large Dutch oven or heavy pot over medium heat, melt butter. Add orzo and garlic and cook, stirring occasionally, until orzo is lightly toasted and garlic is fragrant, about 4 minutes. Add tomato paste and cook, stirring, until combined, about 1 minute. Add broth, milk, sun-dried tomatoes, salt, and pepper. Increase heat to medium-high and bring to a boil. Reduce heat to medium-low and simmer, stirring frequently, until liquid is mostly absorbed and orzo is tender, 12 to 14 minutes.