Hi @Ivan Wilson ,
I could also reproduce this scenario on my end. Each time I migrated a publishing site to SharePoint Online, I found that Page library was skipped as "The list template 850 is not supported" with error code 0x03000004.
As per my research, it is a default phenomenon when using SPMT. The Page library in a publishing site is not a supported list template by SPMT. So, currently we cannot migrate publishing pages using SPMT. I'm afraid you have to ask a 3rd party migration tool which supports it for help. Someone has submitted the feedback here- Enable "Page" library in SPMT. Also, you can upvote for this user voice. It is a place for customers provide feedback about Microsoft SharePoint products. What's more, if a feedback is high voted there by other customers, it will be promising that Microsoft Product Team will take it into consideration when designing the next version in the future.
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