The core contrast is textual before it is thematic: the pasuk itself records that Bereishit 11:31-12:5 has Terach set out for Canaan yet settle in Charan, while Avram alone completes the journey, and the Sforno (Bereishit 12:4-5) makes the point explicit, noting that 'these people with Avram at their head made good on their intention' in a way Terach, who stopped short, did not.
Rambam's account of the origin of idolatry in Mishneh Torah, Foreign Worship and Customs of the Nations 1:1-2 describes exactly the intellectual half-step the question describes: people correctly reasoned that the heavenly bodies serve God and are worthy of honor, but then stopped there and began serving the stars themselves rather than pursuing the reasoning to its true conclusion.
By contrast, Mishneh Torah, Foreign Worship and Customs of the Nations 1:3 describes Avraham as a child surrounded by 'foolish star-worshippers,' including his own parents, whose minds nonetheless kept searching ('his heart wandering and understanding') until he arrived at the way of truth on his own, with no teacher to hand him the answer — the same picture of relentless follow-through versus stopping at the easy answer.
The famous midrashic sequence in Bereshit Rabbah 38:13, where Avraham presses Nimrod from fire to water to the cloud that carries the water, dramatizes the same refusal to settle for the first plausible power, pushing each answer to its next logical implication rather than resting on an immediate, visible force.
The parable of the lit mansion in Bereshit Rabbah 39:1 and its parallel in Midrash Tanchuma, Lech Lecha 3:1 and Akeidat Yitzchak 16:1 present Avraham's reasoning from the world's order to the existence of its Owner as something he arrived at through sustained inquiry, which God then answers directly — the conclusion that idolaters, per Midrash Tanchuma, Lech Lecha 3:1, never got past because they remained with 'their own people and father's house,' meaning avodah zarah, the easier, borrowed answer.
Several philosophical sources elaborate the nature of Avraham's completed inquiry itself: Guide for the Perplexed, Part 3 29:2-3 has Avraham, raised in Kutha, directly confronting and refuting the claim that the sun itself is the ultimate actor; Kuzari 4:40 describes Avraham reasoning, shaping, and refining his inquiry ('וְצָר וְחָקַק וְצֵרַף וְיָצַר וְחָקַר וְחָשַׁב') until it succeeded and God revealed Himself to him; and Mishneh Torah, Foundations of the Torah 1:1-3 and Kuzari 4:15 lay out the deeper metaphysical conclusion - a Being whose existence is independent of all else - that this completed inquiry was meant to reach, beyond the easy identification of God with any visible force.
Later works tie this completeness of Avraham's inquiry to his completeness of character and devotion: the Zohar in Zohar, Lech Lecha 88a says Avraham 'sought to draw near to God, and he succeeded,' surpassing all his contemporaries who remained obstinate of heart and far from righteousness; Sefer HaIkkarim, Maamar 4 41:10 describes Avraham's love of God as so complete that he was willing to forgo all material reward, including the very inheritances promised to him, for the sake of eternal closeness; and Nechemiah 9:7-8 likewise credits God's choice of Avram to having 'found his heart true,' language that reads his selection as a reward for a completeness of commitment.
Among more recent thinkers, Ein Aya, Berakhot 1:77 describes the idolaters as people who sensed that there must be some moral order governing reality but, lacking the fuller insight, fragmented that order into various partial deities tied to particular circumstances - until Avraham came and proclaimed the One God as sole Master whose single will accounts for all providence, completing what idolatry left split apart; and Michtav MeEliyahu in Michtav MeEliyahu (Rav Dessler) describes Avraham as having independently reached completeness in his primary character trait of chesed even before God revealed Himself at Charan, with his later trials developing the rest of his character to match.
Other Acharonim sources extend the theme: Or HaChaim on Bereishit 12:1 explains that Avraham was the first person since Adam to recognize God's divinity and be addressed by Him, which is why God tested whether he would carry out His decrees before revealing His presence; the two passages of Akeidat Yitzchak and Akeidat Yitzchak describe Avraham as needing more trials than the other Patriarchs precisely to fully depart from his astrological upbringing and reach complete love and awe of God, though even he did not attain the ultimate human perfection reached by Yaakov; Daas Torah (Rav Yerucham Levovitz) argues that only by breaking through the final test of the Akeidah without hesitation did Avraham prove that his fear of God held no residual 'tolim,' no contingent attachments, but stood complete and permanent; Rav Samson Raphael Hirsch characterizes the pagan conception of gods as hostile powers one must struggle against or appease precisely because the idolater throws himself upon his own strength rather than pursuing the fuller truth; Alshich records a tradition that Terach himself eventually did teshuvah, suggesting his incompleteness was not final but could still be completed; and Tehillim 119:106 expresses the ideal of swearing and fulfilling 'מִשְׁפְּטֵי צִדְקֶךָ' to its completion, the value these sources see embodied in Avraham's life.