[tripletao]

Vaccines designed at a genomic level target the spike because that's the protein at the surface of the virus, and thus believed to be the most effective target for your immune system. That belief seems to have been correct (at least as to infection), since mRNA vaccines against the spike had the highest efficacy. The spike is unfortunately also under heavy selective pressure for that same reason, leading to the rapid evolution that you note; but that's the inherent and unavoidable tradeoff.

Inactivated virus vaccines (like Sinovac) don't target the spike, since they don't target anything. They do induce anti-nucleocapsid immunity, but there's no evidence that corresponds to any clinical benefit.

I don't think the word "immunogenic" means what you think it does. Vaccines are supposed to be immunogenic (i.e., to generate an immune response); if they weren't then they wouldn't do anything. I've seen papers proposing that the spike protein is toxic in certain cases, but no evidence that the exposure from the vaccine minus the exposure from averted infections is net harmful.