Didn't Jesus return to Earth after the resurrection when he appeared to Paul?

If you take a year's leave-of-absence from work, it doesn't mean that you are forbidden to visit your workplace. During that time, when your co-workers refer to when you are going to return they will be referring to the end of the year, not to an occasional visit.

Similarly, a simple return to Earth, appearing as a physical man, should not be confused with the event known as "The Second Coming".

Consider what happened on the Sunday morning after the Resurrection:

Jesus saith unto her, Touch me not; for I am not yet ascended …. — John 20:17.
…
Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing. — John 20:27.

In verse 17, Jesus is not to be touched because he had not yet ascended, but 10 verses later he asks to be touched. During that interval, Jesus must have ascended to Heaven and returned. But this was not the second coming, not his "Return".

The "Second Coming", the "Return" that Christians await, is much more than a simple visit. It is the time when Jesus will receive his crown and throne, and reign as King of Kings, in the Kingdom of God (or Kingdom of Heaven).

He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end. — Luke 1:32-33

And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever. -- Rev 11:15

And out of his mouth goeth a sharp sword, that with it he should smite the nations: and he shall rule them with a rod of iron: and he treadeth the winepress of the fierceness and wrath of Almighty God. And he hath on his vesture and on his thigh a name written, KING OF KINGS, AND LORD OF LORDS. — Rev 19:15-16

Christians await his return because those that are saved will rule with him, on Earth, during the Millennium:

And hast made us unto our God kings and priests: and we shall reign on the earth. — Rev 5:10

Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years. — Rev 20:6

Finally, God the Father will take over, ruling a purely spiritual, no longer physical kingdom:

Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power. For he must reign, till he hath put all enemies under his feet. The last enemy that shall be destroyed is death. ­— 1Co 15:24-26

I think you might be overthinking this. Going by NKJV, Acts 9:1-9, Christ did not necessarily "appear" physically. Verses 3-4 only indicate that Paul heard a voice and saw a light, and that light could have been anything

As he journeyed he came near Damascus, and suddenly a light shone around him from heaven. 4 Then he fell to the ground, and heard a voice saying to him, “Saul, Saul, why are you persecuting Me?”

That's not really much to go by, and that brings me to my second point: if this was the second coming, why would Paul not write more about it? Typically, when something in the Bible (or really any other documentary) is important in some way, there will be more than a few sentences explaining the importance. That's not to say this event wasn't important; it is, but for a different reason, and as a matter of fact this importance is elaborated on (at least, more so than anything about the encounter with Christ itself). If we look later in the chapter, we find a part where God explains to Ananias the significance of Paul and why he needs to be healed.

13 Then Ananias answered, “Lord, I have heard from many about this man, how much [b]harm he has done to Your saints in Jerusalem. 14 And here he has authority from the chief priests to bind all who call on Your name.” 15 But the Lord said to him, “Go, for he is a chosen vessel of Mine to bear My name before Gentiles, kings, and the children[c] of Israel. 16 For I will show him how many things he must suffer for My name’s sake.”

This is the significance of the event: not that Christ appeared (as he has many times to many people since then), but that his appearance changed someone's life and turned them into one of the Lord's most influential followers, if not the most influential.