Yes, your own proposals make perfect sense. Regarding the opponent mind, they probably try to get an idea of the opponent idea e. g. 'Why does he play this? Normally he goes for the Nc3-variation. Maybe he has found something in the game i won against X. Should I avoyd it, or do I want to se it? "In addition they think a lot about transpositions (Can they by a clever transposition lure the opponent into unfamiliar grounds? Or are they self being lured?).
Why do grandmasters think so much during the opening?
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In top tournaments with very long time controls grandmasters will often take a significant amount of time even on the very early opening, even in the most common openings, just to make book moves that they have certainly made literally hundreds, perhaps even thousands of times during their careers. Moves that they have probably analyzed in extreme detail in the past and could probably give an hour-long lecture out of the blue because they know these moves and all the variants so well.
So why? Why do they take this much time even on these trivial book moves, even the ones that have no significant complications in them, and which they know like the palm of their hands?
Are they, like, deciding on what kind of game they want this time? "Today I'm feeling like I want a queen's gambit declined. I haven't played that one in a while, so it would be a refreshing change of pace"?
Or are they, like, trying to read their opponent's mind and come up with a surprising move that they aren't expecting? (Something that's pretty unlikely to happen, especially with the most common book moves.)
What exactly are they thinking? Certainly they aren't reading ahead, because they probably have memorized the entire opening book for that line. Or at least that's what I'm assuming.