Hi @DuncanRitchie,
Retool recently published an update that may be helpful for your use case, especially if you're working with Custom Components in the new App Builder:
The post covers recent improvements to the Classic App conversion process and includes updated guidance that may be relevant when migrating or adapting Custom Components for the new App Builder.
It might be worth exploring this documentation to see if it addresses your use case or provides additional recommendations for working with Custom Components in the new builder.
Hope this helps!