I don't think the insight check is about his low self esteem at all- the writing intent is pretty clear, at least to me, and the breakup dialogue line I'm talking about doesn't seem like a random bluff like the rest of his breakup dialogue ("you'll be lonelier than I am", "You will regret leaving me, more than anything you live to regret" etc, those are clearly very influenced by his emotions), it adds up with what's being told there.
There is an interesting argument that I agree with that might align more with that idea- that is that the degrading yourself line is about still wanting to serve Tav in their possible degradation kink, because why else would they choose to be with the Big Evil Vampire Lord, if not that? But I don't think this is him thinking "oh, I'm so pathetic and worth nothing, I don't deserve Tav, they're so much better than me" but rather "This is who I am now, this is who Tav approved of me becoming by giving me all that power, and they know that being with me won't be a relationship between equals. So if they kneel they're accepting to be demeaned by me. But maybe they like it, and I can definitely give them what they want then. Maybe this is all they want me for, and I can now provide".
Of course our opinions can differ, but I'd say the intent was pretty clear. What we do with that intent is up to the viewer, of course. I also think sometimes we just have to say we disagree with the writer's intent and engage with, say, fanfiction. It's not the case for me right now as I think what they're exploring is interesting, but I definitely disagree with other things the BG3 writers have stated about Halsin and Astarion, for example.

I also think power dynamics are not explored previously because the dynamics are completely different. In act 1 he's trying to manipulate you, and then in act 2 onwards (until the ritual) he's being very vulnerable and is putting off sex completely. Then after the ritual if he ascends the dynamics completely shift. That's bound to make the way he acts towards Tav change.

I think he makes you kneel because Tav kneeling before they're turned wasn't special to him. He is trying to humilliate Tav, perhaps under the idea (correct or not) that they enjoy it, with the added bonus that he likes having that power over someone. That's the entire point of making them kneel wherever whenever. That is how I always saw it, even before this patch. I love the Ascended romance, but I think I see it very differently to most other Ascended fans I see.

Also, to address the Slaver thing- I often see this piece of misinfo (or rather, inaccurate info) being thrown around. I've been playing Early Access since it released if it's important for what I'm about to say, and I've been aware of that article excerpt since I started playing. It's from February 2020, a journalist's report of something Swen said when showing a beta version of EA, 8 months before Early Access released.
We don't truly know if that was actually part of his backstory when EA released, because no actual details about it were in the game. Whatever seems to point to his corrupt magistrate past is still in the full release (His racism towards the Gur getting him killed, his disdain for other slaves) save for some strange cuts like him saying he would've cut Arabella's hand instead of killing her, but the strange cuts have happened to every companion and seem unrelated to a writing vision, since some other new lines were put in like his view on Justice during the Ansur quest.
I'm the first person that would ADORE him being a really really evil corrupt magistrate, I've been advocating for it since the beginning and was excited for the full game exploring that (I was very disappointed) but the truth is we don't know if that was still part of his story when EA released. The artbook doesn't point to him having been a slaver, but does point to other corrupt shenanigans. I will however note that much to my disappointment the artbook seems quite outdated, so that's not a promise that it is the current vision for him.
Also, for whatever it's worth, during Early Access his writer alluded to that Feb 2020 excerpt during an interview saying that it wasn't part of the story anymore. I've also kept an eye on Early Access datamining (which of course doesn't show the full picture, as not everything would be in the datamines obviously) and no traces of Astarion's slaver past were there. There were traces of a different Cazador quest though, very interesting stuff.