Hi @BeeWhoop ,
According to my research and testing ,unfortunately, there is currently no way to migrate a SharePoint site to SPFX, we can migrate existing customizations to the SharePoint Framework, please refer to this document.
In addition. If you want to migrate SharePoint site , as a workaround, you can refer to the SharePoint Migration Tool. For reference: Overview of the SharePoint Migration Tool (SPMT)
Hope it can help you. Thanks for your understanding.
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