You should not try to convince somebody to take a vaccination or to reject a vaccination. Their health decisions are their business.This is literally the problem ...
convince the yet cv vaxed to take the vax because it works ...
but convince the already cv vaxed that they need a booster because it doesn't work.
For me the decision to take a booster was easy. I looked at the benefits and risks of the booster and compared them to the risks of the virus. If the virus does not warrant a vacvine because the statistics are so low of a serious case or death (as the anti-covid-vaxers claim) then the vaccine itself warrants even less concern (as statistically one is over 300% less likely to have an issue with a vacvine than of catching and having serious issues with the virus).
So if the pro-covid-vaxers are right it is best to get a vaccine and boosters. If the anti-covid-vaxers are right it does not matter either way as the risks of a vaccine are so small they do not warrant concern.
Easy decision for me. Either the vaccine helps a lot or it helps a little on the off chance of getting infected.