It seems to me more feasible to engineer salmon (or cows, etc) with no, or severely curtailed, brains. That would remove most ethical issues with meat-eating.

These would essentially be animals in a persistent vegetative state, so they would be a lot like humans in that same state — needing to be individually hooked up to a bunch of machines to keep them alive. Parenteral nutrition and all that.

While that type of infrastructure is certainly possible, there's no clear way to scale it like there is for cellular culturing.

And the result might be pretty disappointing even if you did it. Picture a human who spent their whole life in a coma in a hospital bed — they'd have zero muscle mass, fragile bones, etc.

However, you might be able to get around this by hooking the animal's body up to a synthetic brainstem that sends computer-controlled impulses down the spine – such that you've got e.g. a bunch of brainless salmon strapped in place in a flow tank, whose bodies are constantly being told to breathe, swim forward, open their mouths to capture food when food is released into the tank, etc.

I think that, besides the scalability challenges, the reason this approach isn't more looked into as an avenue of research, is that the ethics of cellular-cultured meat are really clear (there is nothing experiencing pain there; there aren't even nerve cells to sense or relay pain), while the ethics of brainless whole-animal cultivation are... non-obvious, to say the least.