It's all a crap shoot, with different odds for different points. "Extremely rare" does you no good if you are the poor bastard who dies or gets seriously ill. In my posts, I'm talking about the whole population. But if you are immunocompromised, by all means get a booster. Researchers have found that transplant patients on immunosuppressive drugs who failed to mount any antibodies from a second vaccination had a mixed response to a third: Some did make antibodies, some did not. It probably depends on your CD4 T-cell level. It does not require a normal level (from personal experience), but it does take some level.

My CD4s were so low, my doc told me to stop taking an MS drug around December 2019. By vaccination time, they had recovered enough to make plenty of antibodies. Your mileage may vary. You can check your antibody levels by getting a spike antibody test. I would consider getting one before the booster, and then another a couple of months later to see what the booster did. Less than $100 each at Quest Diagnostics. In any event, another booster is extremely unlikely to harm you.

We're flying electric helicopters on Mars yet you can't turn on your clothes dryer in Texas. That's because scientists are in charge of Mars, and Republicans are in charge of Texas.