You can standout on Wix using Wix Code, which allows you to use your programming skills to code you site, your site backend and a dedicated database for the site.

You will not need to write the whole thing from scratch specific to the customer. Instead, just code the extra bits.

Having said that, I think there is a large market for websites designers and coders, and we see an active community on Wix of people who build websites for customers