"Constantly change their minds" seems to underplay the fact that the evidence available has been constantly increasing. Scientists are obliged to change their position if the evidence tends in a different direction from previously, and that's going to be particularly rapid when a new disease emerges and then rapidly spreads. Besides, it's not quite a fair reflection to say that Johnson's every move has been a carbon copy of the scientific advice he's received. The most recent lockdown, and even the first one, are two clear examples: both of them were implemented later than advised. Whether or not such delays were the correct decision remains to be seen, although, if the reason for delay was a fear that the economic consequences would be dire, and a hope that less drastic measures would suffice, events have proven that this was sadly far too optimistic.
I completely agree that Johnson was unlucky that he was hit with such a major crisis in his full full year. I'm not so convinced that he's the best leader when decisive action is needed. The problem with being naturally optimistic, and with being naturally less of a "details" leader, is that both of these are the worst possible traits to have during a major health crisis.