Integrating Shopify Storefront With Homemade Backend

[Framecraft]

So I'm aware that you can build a custom storefront and connect it to the Shopify backend, but can you do the opposite and connect a Shopify storefront to a pre-existing built-from-scratch backend?

For example, if you have a highly functional website with custom variable products and complex pricing formulas, how difficult would it be to integrate that with a Shopify storefront so that marketing teams could be more hands-on with design, layout, navigation, and content?

Or is it better to start over and use Shopify's backend, even if the homebuilt backend is functional with a high level of complexity for custom products and elaborate production system?

Thanks for any insights!