The solution is to use a harmless strain that has gene deletion(s). These sort of mutations are very unlikely to back mutate as the information has been lost from the viral genome.

The harmless strain doesn’t have to spread naturally better than the dangerous strains and it is likely any harmless strain would not spread better. We can artificially spread the harmless strain around so even if it is less contagious than the dangerous strain we tip the ecological balance in favour of harmless strain. This is really how an attenuated vaccine like the Sabin polio vaccine works from an ecological perspective.

The risk of the virus mutating is there right now. It is much less likely that a harmless strain becomes very dangerous than a dangerous strain become very dangerous. This would be a bigger jump in evolutionary space.

Ultimately as the immediate downvoting of my OP suggests this idea doesn’t appear popular. This doesn’t mean that it is not a scientifically valid approach that could work.