My issue with the example you gave implied (at least to me) that it doesn't matter if Diana has jobbed out to Mary recently (and often), but that she has defeated her at least once in the past. Sounds like you are giving the writers and editors a free pass.
Not sure if it works this way, but the Shazam family share their powers so that if more than one is active, then the rest can't access as much of the power as if only one was? If so, why does it seem the writers never use this 'character bible rule' as they should? I don't think I have ever seen Mary Marvel at nothing but peak power regardless if her 'brothers' were there or not. Of course, if Shiva teaches Mary Marvel how to fight over the course of months it makes any of the lifelong training that Wondy has as DC's best melee fighter to get thrown right out the door.