Perhaps this is because of my lack of interest in the kisses looking particularly good, so I've never really cared about the Spawn angles, but I have to say I don't really feel Spawn was neglected in comparison to AA or anything. Yes, there is a disparity in the amount of kisses, but other than small details like that (the other thing I can think of is AA having like 3 different "What are we to you" responses, and the much needed conversations around Tav becoming a Spawn- it makes sense those are included and they aren't for Spawn, and Spawn also gets special stuff AA doesn't have like responses to half-Mindflayer or Durge Tav) I don't really see one being favoured over the other by Larian?
I also felt that, in terms of narrative, the story bends over itself to say that Spawn is the "right" path (in terms of that being his good ending), to goofy degrees that detract a bit from the dialogue, so in that sense he was the one that felt favoured to me. Like, going through that path, I was like "wow, the writers have a strong bias for Spawn and that might not be for the best sometimes". I don't know if I'm right in that assessment, but that's what it felt like to me.
I'd also say the Cazador aftermath scene is so, so much better for Spawn (the way I describe it is that the Ascension scene feels like a super epic power fantasy where you're like "Whoaa that was cool", but you won't be thinking that much about it afterwards, while the crying scene shakes you to your core for a while in how well it's done and how raw it is) but that's also not really the writer's intent, that was Neil improvising and thinking it felt right to do that, otherwise the scenes would be on much more equal footing.

No one has brought it up here, but in a similar vein, I find it weird when people say Astarion himself in his entirety is hated or neglected by Larian, when he's clearly extremely favoured in terms of being given content. I don't complain because I like him, so I'm "privileged" in that.

Well, that is all to say, I don't think it's a priority either that Spawn gets reworked and/or new kisses.