I think you make a good point here about the line of demarcation between isms and prejudices. Isms are acted upon and prejudices, which we all have, are part of our (mostly) internal mental landscape. They're thoughts we can choose to change, not act upon, or follow up with better thinking in 2nd and 3rd thoughts. @GarrulousGlaswegian wrote a lovely essay on the importance of 2nd thoughts recently.