Let's start by reframing your thinking: which business problems can you reliably solve for companies that value time more than money by leveraging web development?

If you already have a full time job, you might be disallowed from side income derived from services (i.e. consulting). If you don't have such a provision, you can start by selling your services for web development to companies that need them. Search through patio11 and tptacek's comments to read more about this route.

In my opinion consulting is easier than productized software (even if it's less straightforward sometimes). But you can more easily scale up a software product and reduce your time commitment for it. So if that appeals to you, then you should start by looking at what domain knowledge you have.

Now we're talking ... so what sort of web dev do you do? For a business, in an agency, for your own clients? It's all about solving problems. What problems have you solved for them or yourself that you can automate?