How to reduce the sourness of homemade frozen 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