I've heard he did want to submit a version of the tales of the Silmarillion WITH the Lord of Rings (so people could understand the ancient background mentioned)--as one release--in the late '40s, but the whole set was considered two bulky and also there was a paper shortage due to the war, which is why the Lord of the Rings ended up becoming a "Trilogy."--He never intended it to be three books. He also submitted versions of the Silmarillion during the late 30s but it was considered too ''clunky'' by publishers.
As you said, he never quite ''finished'' it--But I believe he wrote he didn't think he would ever definitely finish them. They were always, from 1917 through 1973 always in some state of evolution or change.
It didn't help that he would start writing them, then scratch out what he wrote, and start writing them again scrom scratch. According to HoME there are quite a few different versions in varying degrees of "completeness". HoMe has portions of each of these versions including the vary early, "Book of Lost Tales" versions. Chris Tolkien basically ended up taking what he could from each version, that fit the best, and used those to create most of the material in the released Silmarillion. He and another author "filled in the gaps".
He did get some of Silmarillion material in with the Appendix. But his appendix was to be even longer. I think some of the extended Appendix material appears in People of Middle Earth in the HoMe series.
BTW, the Lost Tales is probably contains the most complete version of each of the stories, in a mostly novelized form. Its unfortunately incompatible with later books due to changes in made in Hobbit, and LOTR to his universe (for example complete rewrite of the Dwarves personality, they used to be evil), and some of his later rewrites. Those later rewrites however only took the stories a few chapters, or half way at the most.
There's actually two other ''full'' stories besides the Children of Hurin.
There are actually three-four as I remember. One is done in an epic poem style. There is also an epilogue of sorts which as I recall, following Hurin after the death of his children. These various versions appear in Silmarillion, Unfinished Tales, and HoMe Series. Most of this material was pulled together to create the released Children of Hurin, plus additional material that Chris says he discovered.
Gondolin as you mentioned and also: The Lay of Lethian - the tragic tale of Beren & Luthien. Apparently those two other works are in more or less a ''complete'' state, and could be published as such--Who knows.
Well most of the version of The Lay of Lethian is actually in the HoMe series. Gondolin's material is split amongst several sources. Including early Lost Tales version, a version in Silmarillion, a version in Unfinished Tales, and maybe a couple of others. There are some major conflicting material however, as Tolkien revised certain events several tims, and would take some editorial tweaking to get merge them into a coherrent story (which is what one author did with the limited run, "Tale of Gondolin").