If the only evidence you have that your software is maintainable is that a company once asked you to come back, and you have no actual experience maintaining large applications with millions of users, you essentially have data to base any of your claims on.

You may have 30 years experience architecting new applications, but when it comes to maintaining large applications, you’re a neophyte.

If you don’t have first hand experience with what long term maintenance looks like for your creations, you don’t have any reason to be telling anyone how to write maintainable software.

If I were you I’d be suffering from imposter syndrome big time. What if you’re just a really good salesman and bullshitter? If I were you I’d want to stick around at a few places to see first hand how my designs hold up.