Thank you for your thoughts, Forel.
Yes, as you say, horary is different than natal. The opposition in horary points to a coming together that does not stay together. As I understand it, all aspects in a relational horary chart between the primary significators point to some kind of coming together: a favorable coming together in horary is the conjunction, sextile and trine; an unfavorable coming together is the opposition and the square. These are all generalities which will vary based on other things like essential dignity, reception, domicile of the signficators, and other variables as well.
And what you say is true in natal/synastry about opposites attract. I am so clear how you connect completion with the opposition aspect. With the Moon and Sun, the opposition is the Full Moon, which is not completion in the lunar cycle; rather the completion is at the conjunction, the New Moon.
I would also mention that opposites are open enemies/7th house, so not always so easily coming together and complementing.
I would think that in synastry, trines ,for example, between suns and moon are preferred and more favorable than oppositions?
Yes I agree
But you know both Lords are highly dignified, the opposition between them is sort of natural. And also there is moon aspecting the Lord of 7th by trine, which many see also as a motivating positive factor when looking the answer 🤷🏻*♂️