I totally agree that you don’t have to pray the rosary or any other private devotion (and there are hundreds!).
Having said that, I too was a Protestant and am a convert (actually reconcilee since I was baptized in the ECUSA). For many years I was in the Assemblies of God and was rabidly against praying the rosary or asking the intercession of the saints, so I have some idea of where you are coming from, although no doubt your experience was somewhat different.
For me praying the rosary was an exercise in practicing the Communion of Saints, since I had been so very much against it, praying the rosary was a “cure” for that mind set. Praying to saints is like asking a family member to pray for us, only these family members are devoid of sin, cannot do anything against God’s will, and are completely in harmony with Him, having been purified of all self-interest and holy as He is holy. So, by not asking for their intercession and help you are really robbing yourself of a great resource.
Also, the rosary is more about Jesus than Mary. It is the verses from Luke pertaining to the Annunciation and the confirmation of it by St. Elizabeth. The only part that is actually a prayer to Mary is the second part. And even it contains a statement of dogma.
The rosary is a “mini-Gospel” and an affirmation of the Virgin Birth, the Divinity of Christ and the Hypostatic union of Jesus’ natures. So, the rosary is more than a nice pious devotion–and why it has been such a vital part of the prayer life of the faithful for many, many centuries.