How can I thicken my smoothie without frozen ingredients?
My favorite smoothie thickener is chia seed.

A tablespoon will thicken a blender of smoothie into pudding. According to the internet it is also a low FODMAP food.

It is a small, black seed which will change the texture a bit. With raspberry or strawberry or vanilla they are lost among the other seeds.

It has the added benefit of being neutrally flavored and high in fiber, protein, omega 3 fatty acids, and various minerals.