As others have said, it doesn't sound like you're getting much of value. At this point, you might even ask instead for a big fat one time paycheck, if that's possible.
But, since you asked specifically what you could possibly do with the left over code, I believe Shopify has a market of add-ons, so I would look to see if you can't salvage some of the code as a Shopify add-on and sell it there. Maybe your ex co-founder can even become your first customer of it.
Yeah, spin up a bunch of modules for different ecommerce platforms that integrate with your platform. See if your ex-co-founder will be your first customer, if they still need the service the software provided. It might actually turn out to be a fruitful relationship to have such a close feedback loop for shaping the software into something customers want. Just be careful not to accidentally build them a custom plugin only useful to them specifically. Charge them accordingly if they ask for big features. A lot of customers to SaaS products are willing to help fund feature development as custom work in addition to the recurring payments..
But, since you asked specifically what you could possibly do with the left over code, I believe Shopify has a market of add-ons, so I would look to see if you can't salvage some of the code as a Shopify add-on and sell it there. Maybe your ex co-founder can even become your first customer of it.
https://apps.shopify.com/