I am not really able to understand whether the basic assumptions of linear regression are met (I'm kind of a novice).

I have a dataset that is big enough (as far as I understand), a series of significants coefficients (both individually and togheter) and an adjusted R-squared over 0.9.

Still, I'm afraid that the model does not fit the data (I tried to use log or quadratic form to improve the situation but I did not get such a big improvement).

I attach the residuals plot, looking for some suggestions. Thanks