A. About the first question (semi/annual booster vaccine), it will depend on the behavior of SARS CoV-2. Time will tell. There are several possibilities: 1. If the pandemic virus behaves like SARS-1, it will disappear from the map. 2. If it does it like MERS, it will give rise to periodic local outbreaks. 3. If the virus "decides" to act as a respiratory agent, it will be seasonal among children and young people. At present, it is an unknown. We must assume our ignorance with humility. Because, paraphrasing Bob Dylan, the answer is blowing in the time.
B. I do not quite understand your proposal about the Spike protein. Vaccines (all platforms) go in that direction: to generate antibodies against the spike. But we should keep in mind that some of the new variants can alter the spike's structure. So, the effect of the immunity can be lesser. In the three arms: natural, vaccine, and, in some cases, therapeutic monoclonal antibodies.
Important note: I am neither a virologist nor an epidemiologist. I am a simple infectious diseases specialist. Thus, my responses to your questions are no more than a particular opinion. Like the philosopher-emperor of Rome, Marcus Aurelius Antoninus used to say, "All is opinion."