Have you actually tried Shopify? I have developed multiple custom theme and apps for my Shopify store clients, you can customize the theme to whatever you want

Have you actually tried Shopify? I have developed multiple custom theme and apps for my Shopify store clients, you can customize the theme to whatever you want using HTML/CSS/JS. And in case of vendor lock in, you can always export out the theme file before switching to another ecommerce store provider, then edit the theme file slightly to match other provider. If you just want an API to accept order and checkout and want to create the whole UI by yourself, you can consider using Shopify Lite for $9 / Month. The bloatness is there for a reason, you will start to regret your decision to go lean after you realize you need to add product variants, custom discount scheme, or even handling chargeback / return /refund down the road. I made a living by helping merchant who stumbled the roadblocks mentioned above to switch to Shopify.