This was a great post. I've been thinking a lot about the issues you've raised here and something which occured to me is that, I think the biggest problem surrounding CC, be they FCC or SCC is that it feels as though they don't bring anything to the group dynamic. They feel like a sounding board off of which the OCs bounce a lot of the time. They don't feel like they add anything special to the story. There isn't really anything they do that it feels like an OC couldn't do as well. And that, combined with the fact we now OCs will get unique interactions with the story makes our CC feel flat.
Thus far Gale is the only OC whose story isn't directly tied to the main plot in some way; Asterion is grappling with the fact that the tadpole has given more freedom and power than he's had in over a century, Wyll's patron has been captured by the Mindflayers (which by the way is a genuinely cool stroyline to go with for a Warlock), Lae'zel's part of a culture that lives to kill Mindflayers, Shadowheart stole an artefact that clearly is important to the plot. The fact that there's Netherese magic at play with the tadpole also leaves the door open for Gale to have sometie to the tadpole as well. But our Tav can't even have the option for that kind of tie to the main quest. Not outside of our own headcanon anyway. We have no way of establishing that our character has a life or anything they care about. We don't get to talk about our past or what we hope for our future. Our character is confined to only expressing thoughts and feelings about the present. And that's kind of an alienating feeling. The Origins have real, driving goals that they talk about and that we the audience can care about. We don't have anything similar that can make them care about us.