Newsletter performance is looked at when it is bad, which means the program only ever learns from failures and never notices what worked.
Writing it down is most of the value
Everybody looks at the numbers after a send. Almost nobody writes down what they concluded, which means the conclusion is gone by the next send and the same lesson gets re-learned quarterly.
A short standing entry per send accumulates into the only real record of what this list responds to, and that record is worth more than any individual week's analysis.
Two separate questions
Whether people opened is a question about the subject line and the sender. Whether they read, clicked and replied is a question about the content. A send with a great open rate and nothing after it is a subject line that overpromised, which is a completely different lesson from a send nobody opened.
Against the plan
The retro is written against what the calendar said this send was for. An awareness send judged on clicks to a booking page is being judged on the wrong thing, and that mistake is what leads to a newsletter that slowly becomes all conversion sends.
Runs well with
Skills compound. These are the ones we usually install alongside it.