Though I'm happy to hear that they have the recordings for the epilogue, but good gracious, it seems that the whole thing is about Karlach... it seems like all you hear is Karlach this and Karlach that. What about if Gale was to romance Shadowheart, or Astarion to romance Laezel, all you get is a genetic "your partner" and a half a sentence afterwards... seems like some origins get a whole lot more voiced permutations, others get a short end of a stick. I guess we all supposed to romance Karlach to get more then one sentence in the epilogue

The reason why you're hearing more about Karlach is because she and Wyll are the ones that need the most work. Astarion has a long, fleshed-out questline. Shadowheart and Lae'zel also have long, meaningful, and story-aligned quests. Even Gale has more going for him where his personal questline is concerned.
Yes, maybe their conclusions might be a bit dry, but at least the ride there was stellar. Meanwhile, if you're following Karlach's story, you get to watch the woman you love die slowly as you sit there unable to do anything but two fetch quests. And then you get to watch her turn into a squid, explode, or you get to light one up in hell.
You must understand how unfair this feels to us. If I could give you a better romance for your favorite character in exchange for a better storyline for her, I would do that swap in a heartbeat.
Just to be clear, I'm talking about the cut epilogue voice lines here and not about Karlach or Illithid in general.
Karlach's lack of story probably comes from her being added so late in the development, hence bad writing and implementation of her quest line. But, honestly I do not know, I've never paid any attention to her, so I do not have anything to go by. As for Wyll, he seems to be even more boring now then he was during the EA.
But in any case, I'm sure you are correct about their development being unfinished, unsatisfying and wanting. But that really doesn't mean that the rest of the epilogue has to suffer. There is literally no mention (I may have missed it of course) in the lines of any origin character by name in romance with another, except Karlach. Are we only supposed to romance her? Is she the only one that gets to be named and the rest gets a generic spouse/partner, I hope not.
Anyway, at this point all we can do is wait until the epilogue is out. We'll see how it turns out, it may be the greatest epilogue ever!