Cook the pasta in salted water according to the package directions until al dente, usually 10 to 12 minutes. Drain thoroughly, rinse under cold water to stop the cooking, and let it cool completely.

Cook the bacon until crisp, transfer it to paper towels to drain, and crumble once cool enough to handle. Chop or shred the cooked chicken, halve the tomatoes, dice the red onion, and slice the green onions if using.

Whisk the mayonnaise, sour cream or Greek yogurt, ranch seasoning, black pepper, and optional lemon juice in a bowl. Add the milk gradually until the dressing is creamy and pourable rather than runny.

Place the cooled pasta, chicken, bacon, tomatoes, cheddar, and red onion in a large mixing bowl. Add the green onions if using, reserving a little bacon or cheese for topping if desired.

Pour most of the dressing over the salad and toss gently until every ingredient is coated. Keep a small portion of dressing aside because the pasta will absorb some as it chills.

Cover the bowl and refrigerate for at least 30 minutes and up to 1 hour before serving. Stir well, add the reserved dressing if needed, and fold in the diced avocado immediately before serving.