WooCommerce Boutique theme update

When running System Status in WooCommerce it shows my current theme, Boutique, is version 1.1.2 but update 1.5.1 is available. Since Boutique is a child theme of Storefront, I have kept Storefront updated, but I don’t want to update Boutique and lose my customizations.

So I’m confused whether I need a grandchild (?!) theme of Storefront/Boutique so that I’m not manually copying changes across to a new version of Boutique. What am I missing here? What is the best way forward?

Thanks,
Peter