The challenges of a vaccine are not because the virus has mutated into different clades, but because of overall technical issues that researchers developing any coronavirus vaccine have faced.
Influenza vaccines are made using three or four different strains of influenza, but are effective against a number of different clades, as long as they are still fairly close to the original strain.
But if this is a “GoF” virus could it be that it attenuates faster than a “normal” virus? Maybe that would explain why there are so many less lethal mutations so quickly. I doubt that would happen with a virus like measles but Coronavirus are known wimps.
But of course there does exist naturally acquired immunity for Influenza Type A and B, which would be protective against the four strains of influenza that are present in the vaccine (generally 2 Flu A strains (H1N1,H3N2) and 2 Flu B strains dependent on the season. But no health professionals are ever interested in testing individuals for the presence of these antibodies before recommending the flu vaccine. What other pharmaceutical do you know of is recommended without first knowing whether it is necessary?