According to the Comprehensive Aramaic Lexicon Project (main website located at http://cal1.cn.huc.edu), the Aramaic word פון found here in this verse was a later editorial insertion, which means that the word appeared above or below (or in the margin of) this verse among the traditional transcriptions of this Targum. In fact, according to the same source, there is no nuance of “the subjective mood” in the Targum Neofiti, the Pseudo-Jonathan, nor in the Samaritan Pentateuch.
Why then did some later editors insert the word פון into this Targum? The following is my own translation of Exodus 3:19-20 from the Aramaic of this Targum. The word
in grey would be the translation of the פון as intended by the Jewish editors who inserted this word in order to bring out the full meaning.
19 And it is revealed before me (lit., was uncovered before me) that the King of the Egyptians will not let you go not even from before his strong army. 20
notwithstanding I will send the stroke of my might, and I will strike the Egyptians with my miracle (lit., my sacrifice [of the Passover]) to perform among them, and thus after this he will cast you away.
Verse 19 starts out with the Aramaic phrase “וּקדָמַי גְלֵי,” which means that the LORD is making the inviolate decree that Pharaoh is not going to budge -- the literal idea here is that there “was disclosed (or revealed) before the LORD” in predictive prophecy to Moses that Pharaoh was going to remain intransigent until the bitter end. Because of this inviolate decree of this predictive prophecy, verse 20 includes the editorial word פון, which tips the reader to understand
notwithstanding, which lends to the idea that while Pharaoh will be forced to release the Israelites, it will not be as a result of his desire to release them. In other words, Pharaoh will not change his mind
notwithstanding that the Passover miracle is going to occur, which will devastate the Egyptians.
Along this same line of thought, in their Commentary of the Old Testament - online, the 19th Century Hebraists Karl Friedrich Keil and Franz Delizsch make the very same observation concerning the Biblical Hebrew of the Masoretic Text.
וְלֹא בְיָד חֲזָקָה, “not even by means of a strong hand;” “except through great power” is not the true rendering, for וְלֹא does not mean ἐὰν μὴ, nisi. What follows,—viz., the statement that God would so smite the Egyptians with miracles that Pharaoh would, after all, let Israel go (v. 20),—is not really at variance with this, the only admissible rendering of the words. For the meaning is, that Pharaoh would not be willing to let Israel depart even when he should be smitten by the strong hand of God; but that he would be compelled to do so against his will, would be forced to do so by the plagues that were about to fall upon Egypt. Thus even after the ninth plague it is still stated (Ex. 10:27), that “Pharaoh would (אבה) not let them go;” and when he had given permission, in consequence of the last plague, and in fact had driven them out (Ex. 12:31), he speedily repented, and pursued them with his army to bring them back again (Ex. 14:5ff.); from which it is clearly to be seen that the strong hand of God had not broken his will, and yet Israel was brought out by the same strong hand of Jehovah.
In summary, the Jewish scholars who translated Targum Onqelos into Aramaic used the nuance of פון to convey nuance in the Masoretic Text that Pharaoh was so intransigent that,
notwithstanding divine power of the Passover miracle to the contrary, he was going to remain in his inveterate obstinacy until the bitter end.