What your view on Travant as a father ? I have seen people opinion range the whole gamut from sweet dad to very abusive, so I was wondering what is your own take on the question. Admittedly, between his interaction with Altena in gen 2 and the whole "planning to marry his two children together", it doesn't paint a very good picture...
It's hard to say given that we don't see his relationship to Arion or Altena outside of the very end, so this is going to mostly rely on headcanon.
Arion's mother (or a mother figure to him at all) is never mentioned during the game, let alone seen. Perhaps she is just considered to be irrelevant enough to not be mentioned, or perhaps she died before FE4 began. I think it's plausible though that Travant is uninterested in marriage (what circumstances led to it being socially acceptable for him to not have a spouse as a monarch in a medieval-era kingdom, I'm not going to think too hard about). I think he just sired an heir for the sake of producing one with some woman, but then chose to raise the child by himself once Arion was weaned. So, at the beginning he saw Arion as a means to an end (of continuing the Thracia bloodline) more than anything else, and the same with Altena when she's taken from her parents. Eventually, I think he mellows out and cannot help but be affectionate towards the two. He isn't incapable of loving them and he tries to be kind. But his ulterior motivation of setting up a unified Thracia still exists.
Overall, I wouldn't say that Travant is outright abusive towards Altena and Arion in the sense of physically abusing them or belittling them like Hilda does with Tine. But as you point out, the fact that he viewed them as means to an end first and foremost set up a toxic relationship, and the toxic dynamic comes out in the way that he treats Altena when she expresses reticence at attacking civilians. If you subscribe to the belief that his end goal was to step aside and have Altena and Arion marry and become co-rulers of united Thracia, that's also messed up in the context of him raising them as siblings. (Sidenote, I headcanon that Travant's story to Altena is that she was the child of random Leonstrian soldiers killed at Aed and that he rescued her out of the goodness of her heart. It doesn't completely explain how she doesn't remember Quan and Ethlyn, but I find it more believable than him making her believe he is her biological father and Arion is her biological sibling.) So he's not literally the worst father ever, but being with him was still not a good situation for Altena or Arion.