Is it possible to only use Shopify checkout only, but design a custom store front

Yes, this is a common use case.

Your developers can use the Shopify Buy SDK to build a headless storefront, that can send your customers to a checkout on a Shopify store, by using the Storefront API to compose a cart.

There are also starters for the various frameworks, like Gatsby and Next (React), that are implementations of this SDK, with more ecommerce functionality out of the box:

Next JS starter (Github repository here)
Gatsby starter (Github repository here)

Hope this helps!