Here is how Daniel Wallace summaries subjunctive with respect to time.
Subjunctive, Optative, Imperative, Infinitive
Except in indirect discourse, time is not seen with these moods. Thus an aorist subjunctive would have a futuristic (or potential) flavor, while in the indicative it would have a past idea. We can say, then, that for the most part time is irrelevant or nonexistent in the oblique (nonindicative) moods.
To sum up: In general, time is absolute in the indicative, relative in the participle, and nonexistent in the other moods.
Wallace, D. B. (1996). Greek Grammar beyond the Basics: An Exegetical Syntax of the New Testament (p. 498). Grand Rapids, MI: Zondervan.
"he has prepared" προητοίμασεν - aorist indicative is past
thus the purpose ἵνα (in order to) took was planned in the past, but
"make known-" γνωρίσῃ - aorist subjunctive was subjunctive because it was yet to happen in relation to "he has prepared." Present subjunctive would tend to indicate a continuous action. While "make known" wasn't a single even, they did occur as a group of single evens. That is snap shots:
What is meant by such verbiage is not that the aorist describes the action as occurring at a point (a notion that gave rise to the “once and for all” aorist), but that the aorist’s mode of presentation is punctiliar. In other words, the aorist takes something of a snapshot of the action. The action itself may be iterative, durative, progressive, etc., but the aorist refrains from describing such intricacies.
Wallace, D. B. (1996). Greek Grammar beyond the Basics: An Exegetical Syntax of the New Testament (p. 11). Grand Rapids, MI: Zondervan.
While at least some events (Esau, Pharaoh) happened in the past, some of the events will be potentially during the second coming.
24, according to which the vessels of mercy are only partly among the Jews. Meyer must also here mix up the second coming of Christ, which he everywhere brings in, just as Dr. Baur does Clemens Romanus. “If, namely, God had not so patiently endured the σχεύη ὀργῆς, but had already permitted His penal judgment to be inflicted upon them (which must be regarded together with the second coming), He would have had no period to declare His glory to σχεύεσι ἐλέους.” That is, the final judgment, as the end of the period of mercy, would have been present with the complete penal judgment of Israel. The destruction of Jerusalem has certainly become a type of the end of the world, but not the end of the world itself. The Apostle presents us with an excellent exegesis of his own language, in chap. 11:11, 25; Acts 13:46, and also in other passages.
Lange, J. P., Schaff, P., Fay, F. R., Hurst, J. F., & Riddle, M. B. (2008). A commentary on the Holy Scriptures: Romans (p. 321). Bellingham, WA: Logos Bible Software.
Up to this point Paul had been speaking conditionally and objectively, but in verse 24 he was more direct—even us—because he and his readers were some of the vessels of mercy sovereignly chosen by God. God not only chose them but He also called them, including Jews and Gentiles. The point is that God’s sovereign choice was manifested not only in the Jews’ ancestry (in Isaac and Jacob, vv. 6–13), but also in Paul’s generation and today. To back up his conclusion and particularly the part about Gentiles, Paul quoted two verses from Hosea (2:23; 1:10). God directed Hosea to give his children symbolic names—one son Lo-Ammi (not my people) and the daughter Lo-Ruhamah (not … loved). These represented God’s abandonment of the Northern Kingdom of Israel to the Assyrian Captivity and Exile (Hosea 1:2–9).
God was not permanently casting away the people of Israel, however. In the verses quoted by Paul God promised to restore them as His beloved and as His people. By ethnic heritage the Gentiles were not God’s people, so Paul was led by the Spirit of God to apply these verses to Gentiles—and Jews also—who were sovereignly chosen by God and called to be His people in Christ. The quotation of Hosea 2:23 is rather free with the order of the clauses reversed to fit the application to Gentiles. Paul was applying these verses from Hosea to the Gentiles, not reinterpreting them. He was not saying that Israel of the Old Testament is part of the church.
Witmer, J. A. (1985). Romans. In J. F. Walvoord & R. B. Zuck (Eds.), The Bible Knowledge Commentary: An Exposition of the Scriptures (Vol. 2, pp. 478–479). Wheaton, IL: Victor Books.
P.S. Pardon my extensive quoting of Daniel Wallace. My Greek education was about the same time he took his. Thus, I didn't have the advantage of his studies as a Greek educator and the insight of his grammar. So, when I first started contributing here, it was with the mistakes he mentions in his grammar. Thus, I learned about Dr. Wallace's grammar here. Consequently, I need to depend on his grammar to update my Greek grammar.