The words πρὸς τὴν μέλλουσαν in front of δόξαν ἀποκαλυφθῆναι gives the sense of immediate future, about to be; going to be. Paul expressed the impending nature of μέλλουσαν as a present participle, and the not yet nature of ἀποκαλυφθῆναι as the aorist passive participle. Note how much the future active indicative ἀποκαλύψω looks like the aorist active indicative ἀπεκάλυψα. Also note that for many verbs the future indicative and aorist subjunctive forms are identical.
μέλλω ... ① to take place at a future point of time and so to be subsequent to another event, be about to, ... ② to be inevitable, be destined, inevitable ... ③ The ptc. is used abs. in the mng. (in the) future, to come ... ④ delay
Arndt, W., Danker, F. W., Bauer, W., & Gingrich, F. W. (2000). In A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed., p. 627-628). University of Chicago Press.
πρός prep. ... ③ w. acc. ... ⓔ to indicate a connection by marking a point of reference, with reference/regard to
... δ. in accordance with ... In comparison with, to be compared to ... Ro 8:18
Arndt, W., Danker, F. W., Bauer, W., & Gingrich, F. W. (2000). In A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed., p. 875). University of Chicago Press.
The key to this [Complementary (Supplementary)] infinitive use is the helper verb. The most common verbs that take a complementary infinitive are ἄρχομαι, βούλομαι, δύναμαι (the most commonly used helper verb), ἐπιτρέπω, ζητέω, θέλω, μέλλω, and ὀφείλω. The infinitive itself is the simple infinitive.
Wallace, D. B. (1996). Greek Grammar beyond the Basics: An Exegetical Syntax of the New Testament (p. 598). Zondervan.