Hi @Matthew Wolcott ,
Per my research, there is currently no OOTB method that goes into preview mode by default.
In order to prevent users from creating new secondary versions, here are two workarounds for you.
Of course, these two workarounds are relatively troublesome for users.
1.Users will be forced to move documents out of the library before making any edits.
Reference: How set Document to open in Preview mode by default
2.Add the Document Library web part and the File viewer web part to the page. Connect the File viewer web part to the Document Library web part and display the selected files from document library through the File viewer.
Display:
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