Will be interesting to hear the views of the experts (and there are always plenty of them on any subject relating to leaning!) But, for the record, this is what I was taught, and it sounds sensible to me:
During the climb, select full rich. This may not be the most efficient way of operating the engine, as you say, but at high power and low airspeed you want all the cooling you can get, and that overrides the efficiency concerns. Lean after levelling off.
During the descent, select full rich. Failure to do so will result in the engine running too lean, and this will cause damage. Lean after levelling off, if appropriate.
FFF
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