Is street noise effectively dampened by a house/garage?

maybe

Maybe, maybe not.

My second question is, since the house is located 250 m away from a highway,

Did you read the linked thread that 11ant found for you
https://www.hausbau-forum.de/threads/haus-direkt-wieder-verkaufen-vorfaelligkeitsentschaedigung.47930/

this noise (relatively quiet

I'll be honest: someone who worries about 3-6 cars or is already dealing with the reduction of "noise," although it is supposed to be "relatively quiet," should move to the woods. There are always present ambient noises, especially neighbors who also live nearby. Complaints, TV noise, music or screaming children, up to playing children, conversations on the neighbor's terrace, dog barking, motorcycles or simply life around you cannot be controlled, but they are there. A healthy person tunes this out.

whether it would also be possible to further reduce this noise through fences/trees in the garden? Or does the highway noise mainly come through air particles/wind, or reflection from clouds?

Bushes are good diffusers and absorbers, trees need time to grow. Noise that is frequency-wise more homogeneous could be "captured" with Helmholtz resonators. Clouds as reflectors is quite a funny idea, no, they rather act like bushes (appropriate punctuation marks here would be short-lived).