Oh wow. This is so awesome. That is a great picture of Luna. Luna works so well for winter. Winter is the time of year when the nights are longer. I associate winter, night, moon and dark together. My character Soram is associated with all of that. Luna works well for that.
I have seen Cadence, Celestia and Luna so far. I guess Twilight Sparkle would be autumn. Part of it is due to process of elimination. Part of it is due to the namesake. Twilight is the time that goes after day and before night. It is like autumn in that things move from light and dark. This is fitting as Twilight reunited Celestia and Luna. Twilight is also the name for the popular book series with sparkly vampires. This is because vampires are nocturnal, and they come out during twilight. Twilight Sparkle is a studious pony. It is the opposite of Cadence, who is more romantic. This can fit. In my country, United States, autumn is when the school year starts. It is a tradition that goes back a long time. In the olden days, children used to work on farms. They cound't go to school in the summer, because they were too busy with farm work. So schools were closed for the summer. Autumn is when thier is a harvest and the work starts to die down. Then the schools would opean for the children. The school year started. Overall winter can be a good time for study and reflection. One isn't so preoccupied with farming, because it is too cold outside. So autumn would be good preparation for that. That is just some food for thought.
I don't remember Celestia and Luna being explicitly linked to seasons in the canon show. It is brilliant that you figured out the connections anyway.