Right, I thought you were RobinZ. By the context, it sounds like he does consider serenity incongruous with heroic responsibility:

I agree. I downvoted RobinZ's comment and ignored it because the confusion about what heroic responsibility means was too fundamental, annoyingly difficult to correct and had incidentally already been argued for far more eloquently elsewhere in the thread. In contrast I fundamentally agree with most of what you have said on this thread so the disagreement on one conclusion regarding a principle of rationality and psychology is more potentially interesting.

With my (rhetorical) question, I expressed doubt towards his interpretation of the phrase, not (necessarily) all reasonable interpretations of it.

I agree with your rejection of the whole paragraph. My objection seems to be directed at the confusion about heroic (and arguably mundane) responsibility rather than the serenity wisdom heuristic.

For me at least, saying something "can't be changed" roughly means modelling something as P(change)=0.

I can empathize with being uncomfortable with colloquial expressions which deviate from literal meaning. I can also see some value in making a stand against that kind of misuse due to the way such framing can influence our thinking. Overconfident or premature ruling out of possibilities is something humans tend to be biased towards.

I would call this "level-headedness".

Whatever you call it it sounds like you have the necessary heuristics in place to avoid the failure modes the wisdom quote is used to prevent. (Avoiding over-responsibility and avoiding pointless worry loops).

By my intuition, serenity is a specific calm emotional state, which is not required to make good decisions, though it may help.

The phrasing "The X to" intuitively brings to my mind a relative state rather than an absolute one. That is, while getting to some Zen endpoint state of inner peace or tranquillity is not needed but there are often times when moving towards that state to a sufficient degree will allow much more effective action. ie. it translates to "whatever minimum amount of acceptance of reality and calmness is needed to allow me correctly account for opportunity costs and decide according to the bigger picture".

My dataset luckily isn't large, but I have been able to get by on "numb" pretty well in the few relevant cases.

That can work. If used too much it sometimes seems to correlate with developing pesky emotional associations (like 'Ugh fields') with related stimulus but that obviously depends on which emotional cognitive processes result in the 'numbness' and soforth.