I realized a left out a few considerations. Could you please have a look at them?
1) The sickle stands for the inevitable passing of the time, whereas the adjective ‘brief’ points out the transience of beauty and time.
2) In the first two quatrains the poet dwells upon the description of the passing of time and the decay of natural things whereas in the third the passing of time is linked to the theme of old age.
3) The sonnet’s meaning is contained, as usual in Shakespeare, in the final rhymed couplet: nothing can oppose time’s destructive power except marriage and procreation.
2. comma after "natural things"
All else is fine.