This is indeed the most plausible yet simple theory for vaccine-induced or -related myocarditis over the more complicated ones out there.

Initially, I expected myocarditis risk to be higher after the third than second dose, given that booster shot would mount stronger immune responses. But a recent study showed that this isn't the case for Moderna. And this could be due to the lower dose used for booster shot. So, the dose of spike protein antigen is likely the key factor in vaccine-induced myocarditis.