I'm aware of one surveillance study (link below) showing that myocarditis risk is highest following the second dose of Moderna mRNA vaccine. Such risk from the subsequent Moderna booster dropped to the risk lower or on par with the first Moderna dose. The lower myocarditis risk from the booster may be due to the halved dose use, I think.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9439633/

But risks are cummulative. Taking three shots still means that the myocarditis risk is higher than just two shots.

I don't have much thought on the death rate of vaccine breakthrough infections, however, as I haven't read into this in depth. But as far as I understand, boosters still lower the risk of severe covid for another few more months.