If it's so easy, then let's see you do it, Daniel. Let's see you outline that informal creed.
It must be remembered that the "creed" -- if it must be so called (Mormonism really is non-creedal) -- is genuinely informal. Which means, among other things, that it won't have a precise form, or exact delimitations. Still, after decades as a committed Latter-day Saint, I have little difficulty envisioning roughly what it is and what it isn't, and have no sense that any significant number of my fellow believers have much problem with the concept, either.
Certain claims are essential. For example, that there is a God, that he has a divine Son (known to us as Jesus Christ), and that, with the Holy Ghost, these three form a Godhead or, even, a Trinity. That they are in human form. That salvation is available through, and only through, Jesus Christ, who took mortality upon himself, was crucified, rose physically from the dead, and, in some incomprehensible way, atoned for our sins as well as for the transgression of Adam. That we -- all of us -- will live beyond the grave (as we lived before birth), and that, thanks to Christ, we too will be resurrected. That we will be judged by God, and assigned to some level or other of glorious salvation (or, in the case of some, to a state of no glory whatever). That the destiny of the righteous is to receive all that God has, and to be "gods." That God called Joseph Smith to be a prophet, and, through the angel Moroni, revealed the Book of Mormon. That unique priesthood authority was restored by John the Baptist; by Peter, James, and John; and, subsequently, by other ancient prophets in the Kirtland Temple. That Joseph Smith established the Church of Jesus Christ of Latter-day Saints under divine mandate, and that the fullness of the saving ordinances is available in, and only in, that church. That the reception of temple ordinances is necessary for the highest mode of salvation. That the Bible, the Book of Mormon, the Doctrine and Covenants, and the Pearl of Great Price constitute the "standard works" or canonical scriptures of the Church. That Joseph Smith's successors are apostles and prophets, and that the First Presidency and the Quorum of the Twelve are prophets, seers, and revelators.
I believe all of these things. More significantly, in this context, I doubt that very many communicant Latter-day Saints of normal intelligence, if any, would fail to recognize these as fundamental and official doctrines of the Church.
Also to the point: Public denial of any or all of these teachings would expose the denier to potential Church disciplinary action.
Now to a list -- I'll make it shorter, because I have a lot of other things to do than to occupy myself with this thread and this subject, particularly over the next few days and even over the next three weeks (after which I'll be gone for about one and a half months), but sufficient to illustrate the proposition -- of things that would almost certainly not involve one in a disciplinary council, regardless of which position on them one took: Coke and chocolate are against the Word of Wisdom. Evolution occurred, perhaps even up to and including the evolution of the hominids. The events of the Book of Mormon took place, principally, within the boundaries of the United States. The biblical book of Job reflects literal biographical history. The scriptural canon is essentially inerrant, with every apparently historical assertion to be taken as literally true and accurate. Talks in general conference are essentially inerrant, and equivalent to canonized scripture. The destiny of the righteous is to be able to create and preside over worlds of their own.
Differences and nuances of opinion exist among faithful, communicant Latter-day Saints about all of these propositions, and even, to some extent, about many of those in the first list, given previously. (You may or may not be able to accurately guess my views on all of them.)
There are, nonetheless, as I've said, unresolved questions about the precise boundaries of what is required to be a Latter-day Saint.
Some would say, essentially, that one has to be a political conservative in order to be a good member of the Church. I am personally a very serious political conservative, trending strongly libertarian (particularly on economic issues), but I believe this to be false -- and the Brethren and other Church leaders seem to be on my side: Harry Reid remains a member very much in good standing, as do many thousands of "leftists" (including Laborites in the UK, Chavistas in Venezuela, and a devoutly Marxist friend of mine in the American northeast).
Some would say that one cannot be a good Latter-day Saint and accept the theory of evolution. But I do. And so do many, many Latter-day Saints, including a substantial contingent of LDS biologists and geologists, not a few of whom have served as bishops, stake presidents, and area authority seventies. Including James E. Talmage and B. H. Roberts.
There is, too, no official Church position on the exact proportion of "nature versus nurture" with regard to the question of homosexuality (or any number of other more or less analogous issues in human behavior).
There are a number of such "border issues," but they are, precisely, "border issues," and do not substantially disturb the communion of the Saints.
I find efforts to depict the doctrinal situation within the Church of Jesus Christ as utterly unstable, wholly in flux, to be overwrought and inaccurate -- and, not infrequently, disingenuous. And, significantly, while I observe such efforts among critics, I don't believe that I've ever encountered similar "concerns" among fully communicant believers.
As to the specific question of how I personally decide what is, and what isn't, a "fundamental claim" of the Church that I should defend, the simple answer is, flatly, that I choose. If I do not regard the issue as important or central, I feel no obligation to spend time on it. (I'm not paid, or directed, to spend any time at all on apologetics; I do it entirely of my own free will and choice.) But I do not think that my choices are, or have been, eccentric. No member of the Church, at any rate, has ever reproached me on those grounds.