I understand that you’re considering to transfer your domains to Bluehost, and you also have one domain connected to a Shopify store. While you can absolutely transfer a third-party domain to Shopify, it also make sense to transfer all your domains to a single provider for billing purposes. Since you can always connect a third-party domain to Shopify, then transferring the domain to Shopify wouldn’t be necessary. It’s also worth noting that Shopify doesn’t offer email hosting, which may be something that’s offered by a third-party domain host. However, Shopify offers does provide email forwarding for Shopify-managed domains should you do decide to transfer your domain to Shopify.
If you have any other questions on this topic, then don’t hesitate to let me know!