but I've got some hangups about our ability to make objective decisions about what is "in something/one's best interests."
Yeah, me too :/ It's like every human (or animal) right - it has to be enforced by people, and people are pretty shitty. I don't think that means we reject the principle, it means we put guardrails around it to try and prevent errors and abuses.
And I certainly agree: lab grown meat is far less heinous and morally offensive than factory farming. It involves a moral compromise for vegans, but, well, so does almost everything else. We can recognize both aspects.
The definition of veganism, from the Vegan Society:
Please note the italics.
Living without modern medicine fits squarely within "not possible or practicable" because you can literally die without it. If you refuse vaccines or treatment for contagious diseases, it's even more compelling, because you're not only risking your life but the lives of others.
On the other hand, it is completely possible and practicable to live without lab-grown meat, so "were animals exploited to create this product" is a much more relevant consideration.