How to launch apparel shop with custom designs

I want to launch an e-commerce business where buyers can do some basic design for custom t-shirts/hoodies/mugs/etc. (e.g. pick an inspirational quote and a position it on an AI-generated background, add some other flourishes). How can I validate this easily, end-to-end, before I throw significant resources at it?

I'm an experienced web developer so I can code the custom designing front-end, but don't have any experience with e-commerce platforms. I could just put up a landing page with the designing front-end, but feel that would not give me the end-to-end experience of running the e-commerce shop, and I've already validated the idea with ~100 people at networking events by wearing said t-shirt myself.

So how would I get started with selling these apparel designs? Could I plug in my frontend code into Shopify? Or use some e-commerce engine via an API? How do I handle fulfillment/shipping without paying a large commission to Zazzle or similar providers? (I feel they're an intermediary between the design and the actual apparel printing service).

Thanks and feel free to point me to n00b resources. I've read this post but am still in the dark.