Hi @William Lawler ,
Per my understanding, you could customize categories.
However currently SPO does not support the display of events with multiple values. Some users have expressed the need for improvement and have made suggestions to the official in SharePoint UserVoice. I would suggest that you can also vote and make your suggestions:
Multiple Categories for Events Web Part
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Please see the dynamic picture below.
I created a new event, the custom category is TEST111 and selected this category in the event web part:
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And you can continue to use this category when you create an event next time, it will be automatically displayed because it has been automatically saved by the SP:
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