The 1874 RLDS edition omitted the relative pronoun that here in “because this is all that thou hast desired”, perhaps because of the awkwardness of this that when it is followed by another that, the subordinate conjunctive that in “that I would release thy brethren”. The 1908 RLDS edition restored the original that to the RLDS text. The relative pronoun that is quite acceptable following all, as in the similar language in Alma 11:46: “or this is all that I have written”. Also see the more general discussion regarding “all (that)” under 2 Nephi 25:23. The critical text will maintain the reading of the earliest text here in Alma 20:26, despite its awkwardness.
Summary: Maintain in Alma 20:26 the relative pronoun that in “because this is all that thou hast desired”, the earliest extant reading.