One really obvious reason why you can't be Catholic without accepting Marian Dogmas is that they're built into the Liturgical Year as the penultimate celebrations.
- December 8th, Solemnity of the Immaculate Conception of Mary
- January 1st, Solemnity Mary, Mother of God (you could stand this one)
- August 15th, Solemnity of the Assumption of Mary into Heaven
These are all Holy Days of Obligation, which means, in order to be a Catholic in good standing, you need to attend Mass on each of these days (unless moved because of Sunday Mass [except for the December 8th, which is always celebrated]).
So, if you think of the Mass as one big prayer, the ones that are Marian focused require your devotion to her; which is to say, whatever you bring to the table in her honor will be multiplied by her Divine Son.
Dogmas are the way the Church makes sense of the supernatural realities found in the Bible and Tradition. If you can figure out a better way to reconcile Sacred Scripture with Sacred Tradition, that would make for an interesting book. But if you delve into it deeply, and continue the path you said you set up for yourself, I think you'll find out what G.K. Chesterton found out when he put the finishing touches on his new heresy.
I did try to found a heresy of my own; and when I had put the last touches to it, I discovered that it was orthodoxy.
Still, as I said in my deleted comments a few days ago, Chesterton didn't become Catholic until well after writing Orthodoxy where he asks, as you do (more or less, he's talking about original sin (which is connected to all Marian Dogmas))
why cannot you simply take what is good in Christianity, what you can define as valuable, what you can comprehend, and leave all the rest, all the absolute dogmas that are in their nature incomprehensible?
to which, he answers himself (this time concerning the Incarnation)
mean that having found the moral atmosphere of the Incarnation to be common sense, I then looked at the established intellectual arguments against the Incarnation and found them to be common nonsense.
And you could insert any of the Dogmas proclaimed by the Catholic Church in place of the Incarnation. What are the rational arguments against the Assumption of Mary anyway? And furthermore, what right have we to doubt the testimony of a Miracle, like the Assumption of Mary, just because it didn't happen in the books the Church recognized as canonical?
Somehow or other an extraordinary idea has arisen that the disbelievers in miracles consider them coldly and fairly, while believers in miracles accept them only in connection with some dogma. The fact is quite the other way. The believers in miracles accept them (rightly or wrongly) because they have evidence for them. The disbelievers in miracles deny them (rightly or wrongly) because they have a doctrine against them. The open, obvious, democratic thing is to believe an old apple-woman when she bears testimony to a miracle, just as you believe an old apple-woman when she bears testimony to a murder.
This is one reason that the Church has always kept Sacred Tradition in the same company as Sacred Scripture. Even the most Bible-thumpingist evangelical preacher believes this when he professes to cast demons out on live television. If a miracle occurs, which I believe happens to Protestants as well as Catholics, then another line is written in Salvation History.
We just baptized our fourth child this morning, and she did a great job of not voicing any dissent when we renewed our baptismal vows for her benefit. She just laid there and smelt good. I think if you can manage to lay down and smell good, you'll make a good Catholic eventually. If you were at Mass this morning, you'd also have read:
Be sure of this: if the master of the house
had known the hour of night when the thief was coming,
he would have stayed awake
and not let his house be broken into.
And that would give you pause to think. In any event, I think, from talking to converts in my Parish that Marian Devotion is so foreign to them that they haven't even analyzed it the way you did. So, the way one probably can approach the matter is to put yourself in their shoes again. If you want to enter the church, I think you have to enter it uncritical of the Marian Dogmas and let her teach you what's up with them. There's no point putting yourself at a disadvantage from the tens of thousands of converts who just did it to make their wives happy and entered the same way my daughter just did. Knowing little and accepting the truth as a child would.