What precautions need to be taken while using sour culture for setting yogurt

Overly sour yogurt is a sign of inconsistent inoculation

Check the type of culture you are using, and ensure you are keeping the yogurt above 37C for more than six hours. The yogurt should finish as a solid lump that resists pouring, not a thick liquid, or a lumpy paste

Pouring off the whey and straining the yogurt makes it more creamy and taste sweeter

Thickeners are not required, just complete inoculation

For sweet yogurt dishes I mostly use sour berries (blackberry, raspberry etc) so it is expected to be somewhat sour anyway

There is no reason why you can't turn chocolate milk into yogurt. Now, much commercial chocolate milk has carrageenan added as a stabilizer. If you get issues with carrageenan separating out, you can also use Torani chocolate syrup (intended to flavor coffees) to add the flavor to normal milk.