I think if the thread gets too deep, HN will cut it off, so I'll to continue in parallel here. I hope you see it so we can continue (if it pleases you).
> what level of consciousness matters?
We can start at least with the obvious distinction between "None at all" vs "Some" even if we don't know where the change happens. Asking "how much" only works if you want to investigate this dilemma within the "some" category, which we at least agree does not include embryos, and that embryos are in the "none at all" category. Investigating the "some" category strikes me as substantially more difficult because of that question about how much is enough.
For now, I say that "some" level of consciousness is required for mattering, which precludes a "none at all" level of consciousness from mattering.
Consider brain death. When a body becomes nothing more than a mass of tissue kept in motion by machine intervention without any brain activity, do we not say that it's ok for the family to decide to pull the plug? Because I say that it's ok to not keep brain dead patients on life support until the sun eventually explodes and the power runs out. But maybe you don't. And I understand that this is talking about an end rather than a beginning, but it's really just the other end of your question about how much consciousness matters. Clearly in that scenario the consensus answer is "at least some".
> There are severely disabled adults who have never been as "conscious" as some of the more advanced other animals. Do they just not count?
Question needed for me to establish a more shared understanding: Are you vegan? (I am not.)
If so, most people in the world are not, and that means if we're comparing to animals then clearly, at least en masse, we do discount lesser animal consciousness even if good vegans don't. But at least this framework would be consistent with a view that a threshold of human-level consciousness shouldn't matter.
If not, do you have a basis other than degrees of sapience for believing that a human embryo is worth worrying about but a chicken embryo is not that doesn't stem from egocentric instinctive queasiness? (I mean "egocentric" neutrally here, not negatively) Because the only other option I know of is "the <religious_book> says so", which is thought-terminating and we won't be able to move past that.
> If this tech had been commonplace when my parents decided they wanted to have a baby and conceived me, they would have had to look at this graded set of their offspring, one of which was me, and pick me out of the group
I get what you're saying but people actually already do this decision-making and have been doing so for a very long time. They just do it ahead of time by choosing who they reproduce with. Couples all over the world make decisions literally all the time about passing on negative potentially-heritable traits, from mild Autism to Scoliosis to Tay-Sachs.