Is there no room to say that just as regarding Israel we do not find commandments about character traits because of how obvious they are (as is well known in the name of Rabbi Chaim Vital), so too the descendants of Noah were not commanded about giving thanks because of how obvious it is?

So why is it different for Israel? Though admittedly this is rabbinic.

What does “association” mean?

Also, the way I understand it, giving thanks for God’s goodness is part of preserving the human image/dignity; gratitude is something moral.

Association is the belief that besides the Holy One, blessed be He, there are other gods together with Him. Gratitude obligates all inhabitants of the world, and obviously gentiles as well. So they are obligated to show gratitude to the Holy One, blessed be He. But gratitude can be expressed in various ways, or even just be felt inwardly. The form of a blessing in our wording is not necessary, and that is imposed only on Israel.
By the way, I once argued that there is a Torah-level obligation of blessing to thank and show gratitude to the Holy One, blessed be He. The rabbinic obligation is only to do so in the specific manner established by the Sages. In that formulation, gentiles are obligated in the Torah-level obligation but not in the rabbinic one.