OpenCart is awful, what are some decent alternatives written in PHP?

Sorry if this is the wrong subreddit, I wasn't sure where else to post it. If this is the wrong place, please point me to the right sub.

I'm helping a friend convert their shop to an actual ecommerce solution - right now they're just using some fairly insecure, poorly written PHP they made themselves (They learned PHP making this). It has several issues that I'd like to fix by using a proper solution.

So after little research, I decided to go with OpenCart - it looked decent enough on the frontend, so why not? Well... Once I started trying to modify it to how he wanted it (Share the main site's theme, try to recreate the product listing he had for his shop, etc.) I ran into so many problems. I can fix them with enough time, but I'm not getting paid enough to spend 20 hours reworking this for what should be minor changes, or features already built-in.

So - what are some good alternatives written in PHP that are easy to work with, somewhat modern, and customizable?