Hi @Zach ,
It seems that you want the subsites to inherit the site master page from the parent site. If so, you may have a try below settings:
- https://www.c-sharpcorner.com/UploadFile/93cb27/change-the-master-page-to-site-in-sharepoint-2013-online/
- https://praveensharepointknowledgebase.wordpress.com/2018/03/05/change-sub-site-master-page-without-enabling-publishing-feature/
Best Regards,
Baker Kong
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