Yes, most vaccines are very specific for a particular original or variant just because of the way the generated antibodies fit on the virus. But any of them activate your T cells, and T cell immunity is much broader. T cells respond to up to 52 epitopes (amino acid sequences that the immune system can "see") on the virus. To evade that response, so much would have to change the virus would not function as SARS-COV2 any more. A recent paper studying people with a genetic condition that prevents them from making any antibodies, found that they had the same clinical response to SARS-COV2 infection as people who do make antibodies. This suggests that antibodies play no part in your body's fight against an initial infection!
We already know that you have to wait a few weeks at least to measure IgM, and even longer to measure any IgG antibodies. Your T-cells have already inactivated all the "live" virus by this time. It looks like antibodies just prevent reinfection for a while. It's perfectly normal for antibody levels to fade. That makes reinfection more likely, but you still have memory T cells and B cells in your lymph nodes and bone marrow that react quickly, And even if it's Delta, Lambda, or Omega, they make a new antibody that is specific for that variant. Antibody levels just tell you your T and B cell chain is working.
New variants seem like they are more contagious because they have slightly better fitness than previous variants. That makes a difference in large populations (like a billion billion virions), but makes almost no difference to individual human hosts. Every transmission event still requires some infected person coughing, shouting, singing, etc. in somebody else's face. We get surges in areas that have low vaccination rates, where the unvaxxed mask refusers get together in crowds. That's EXACTLY what the original virus did! All the media claims that each variant is deadlier than the last is just the result of wider infections. More people end up in the hospital because more people are infected. But that does sell more papers!
A study of Delta variant infected patients found more virus RNA being shed. Seems like a dangerous new variant, doesn't it? The problem is they also ran virus antigen testing on them that reacts to the whole virus nucleocapsid protein, and they found much lower levels than they expected. This means their PCR tests were just showing a lot of virus RNA fragments, but a normal amount of virus being shed. That suggests that Delta is actually LESS fit, since it's replication process spews a lot of bits of virus RNA before it is encapsulated into complete (and infective) virus.