This is just wishful thinking. We've never had effective vaccines for this sort of virus. (The yearly flu vaccine is like 30% effective.) Sure lots of research groups are working on vaccines, but many of them are academics who have no particular duty to work on research likely to have an immediate payoff. The researchers who do have such a duty, i.e. those who work for private drug firms, are mostly developing treatments like the antiviral remdesivir. Effective treatments of various sorts are closer than any vaccine, for COVID-19. IMHO, the most likely eventual winner will be a scaled-up version of the convalescent plasma therapy, which unlike the current version will be able to produce effective antibodies without drawing blood from humans.

Efficacy is always a question, sure, yet it seems the spike protein is stable (conserved across mutations).

If the mRNA stuff works well then it'll likely work for the flu too. (It be easy to pack one shot full if flu strains.)

So, stability over e.g. a month doesn't guarantee stability over longer terms.

Covid (any vaccine based treatment really) is even more difficult as you need to wait for six months to assess transmission.

Technically, you don't. There are volunteers ready to get infected with SARS-CoV2 and I wouldn't put my money on "ethics" standing in the way in this case.