Is this what the OP is arguing then? They are trying to argue in favor of this secular view?
Yes
The time of the Crucifixion is based on historical events.
One is the Passover which the Israelites marked on the their calendar as the date of the liberation from Egypt. It had nothing to do with the equinox. It is the traditional date.
The Crucifixion is based on that date as Jesus had an historically legitimate reason to be where he was, doing what he was doing.
These dates were determined, just like our 4th of July, to commemorate historical events.
The secular view of these historical commemorations being symbolic representions of sun worship is historically inaccurate.
Having been raised in the secular view that all religions are myth, better characterized as "symbolic representions" of natural phenomenon, I see the OP as a core tenet of the secular/atheist belief system but the issue is larger than just Easter.
Easter is a favorite target for the secularist to argue. Easter, to the secularist, is merely continuation of older myths.
The secular view is that religion is evolving. So the secularists attempt to trace the roots of Christianity by drawing false parallels with other belief systems, altering or ignoring historical facts and drawing false equivalencies, such as Easter being sun worship, based solely on proximity.. According to that system the 4th of July is actually a sun worshipper's celebration of the summer solstice.
There are other components of the "religion is myth" belief, the most notable that man creates God in his own image.
In myths that is certainly true.
There are myths that are symbolic representations of natural phenomenon, such as Thor, god of thunder
Thor was also a symbolic representation of man as god, Thor had all the attributes and form of man.
The pantheon is another example. While those god and goddess had "god power", they also had the attributes of man, including all of fallen man's lust, pride, envy, hatred, and fear. They represented what man would want to be if man could be god. Superman!
However, the final argument against the contention that Jesus as only a mere man or the God Jesus represented was only a symbolic representation of man as god is the fact that all the pagan gods are merely human with human attributes and super human power.
Jesus did not fly to Jeruselem. He rode a donkey as a man and yet, as God, He was not the image of man.
Again, the final answer from the Bible itself
Psalms 50: 21 These things hast thou done, and I kept silence; thou thoughtest that I was altogether such an one as thyself: but I will reprove thee, and set them in order before thine eyes.
The secularists believe that God is altogether such a one as thyself, and therefore man created god in his own image.
God is altogether not such a one as man
Man could not concieve of the true God without Jesus Christ to set them in order.
It would be like a man trying to descibe an elephant when he doesn't know a thing about elephant.
That leads to another interesting thought.
In all of history man has known he is in the presence of God, however faulty his representations of that Being have been. Only through Our Lord can we truly know God.