Whether Mary is necessary to the spiritual growth of a Christian

Objection 1. It would seem that the spiritual life of a Christian could be equally as rich without the admixture of devotions to the Blessed Virgin Mary. Our Lord, as the one Mediator is sufficient for salvation and therefore all the spiritual growth one needs is wrapped up in salvation as one doesn't need more than salvation to be saved

Objection 2. Devotions to the Blessed Virgin Mary is always to be figured secondarily to true devotion to Our Lord. The Lords Prayer is a complete summary of the life of prayer, whereas the Hail Mary is just praise followed by petition.

Objection 3. The Apostolic Fathers, especially those who knew Our Lord best; as well as Our Lady, would not have had recourse to the devotions we have today because those devotions developed over the course of centuries and those devotions were directed through them which would have been an odd sort of devotion for someone who personally knew Jesus.

On the Contrary. St. Louis De Montfort states in True Devotion to Mary:

ALL our perfection consists in being conformed, united, and consecrated to Jesus Christ; and therefore the most perfect of all devotions is, without any doubt, that which the most perfectly conforms, unites, and consecrates us to Jesus Christ. Now, Mary being the most conformed of all creatures to Jesus Christ, it follows that, of all devotions, that which most consecrates and conforms the soul to our Lord is devotion to His holy Mother, and that the more a soul is consecrated to Mary, the more is it consecrated to Jesus.

I answer that Mary is a model for the Church. St. John Paul II refers to Mary as a model for the Church sixteen times in his encyclical Redemptoris Mater

Mary is present in the Church as the Mother of Christ, and at the same time as that Mother whom Christ, in the mystery of the Redemption, gave to humanity in the person of the Apostle John. Thus, in her new motherhood in the Spirit, Mary embraces each and every one in the Church, and embraces each and every one through the Church. In this sense Mary, Mother of the Church, is also the Church's model.

https://www.vatican.va/content/john-paul-ii/en/encyclicals/documents/hf_jp-ii_enc_25031987_redemptoris-mater.html

Without her model as a prototype, man is bound to flounder, creating his own religions fashioned off his own vices. He may be lucky, like G.K. Chesterton, to found his own heresy and discover it was orthodoxy, but much more likely he'll found something else.

Reply to objection 1. St. Louis explains the purpose of following Mary to Jesus,

It is an easy way. It is the way which Jesus Christ Himself trod in coming to us, and in which there is no obstacle in arriving at Him. It is true that we can attain to divine union by other roads; but it is by many more crosses, and strange deaths, and with many more difficulties, which we shall find it hard to overcome. We must pass through obscure nights, through combats, through strange agonies, over craggy mountains, through cruel thorns, and over frightful deserts. But, by the path of Mary, we pass more gently and more tranquilly.

Being an "easy way" doesn't mean it is always comfortable, but Catholics who do these things believe that certain prayer and devotions have certain "signal graces" attached to them. For instance, one who perseveres in praying the Rosary may not die without last rites (one of the 15 promises of the Rosary).

6. Those who recite my Rosary devoutly, applying themselves to the consideration of its sacred mysteries, shall never be conquered by misfortune. In His justice, God will not chastise them; nor shall they perish by an unprovided death, i.e., be unprepared for heaven. Sinners shall convert. The just shall persevere in grace and become worthy of eternal life.

https://dominicanfriars.org/the-15-promises-of-the-rosary/

Reply to Objection 2 The Lords Prayer is the summary of Christian Prayer, but a prayer through an advocate, a most gracious advocate (as St. Bernard might say) is an effective tool at making your case before God.

... in heaven and on earth everything, even to God Himself, is subject to the Blessed Virgin, they mean to say that the authority which God has been well pleased to give her is so great, that it seems as if she has the same power as God, and that her prayers and petitions are so powerful with God, that they always pass for commandments with His Majesty, who never resists the prayer of His dear Mother, because she is always humble and conformed to His Will.

True Devotion to Mary

Given Mary's intercessory power, Catholics should pray Hail Mary with Our Father as much as possible, which is a very natural thing to do in almost every Catholic devotion (Rosary, Divine Mercy Chaplet, when Praying for the Pope's intentions, etc...), avoiding the Hail Mary is about as fruitful as avoiding pants.

Reply to Objection 3 The apostolic Fathers may not have recourse to the Blessed Virgin's intercessory power in Heaven until her Assumption, but they certainly had recourse to her earthly power, grace and wisdom. If the stories be true, and she was Conceived without Original Sin, then her prudence would have been infinite. Failure to turn to her in trouble would have been foolishness.

Now, as to the rest of the saints, I could copy and paste the above and make it to a lesser extent, but Mary is the model for all the saints. It is possible to be consecrated to St. Joseph, have a devotion to St. John the Baptist, or a saint whose life touches yours in a personal way, but Mary (and Joseph) are the Universal saints whose intercession everyone who ever was can understand, benefit from and grow in love of Christ through.