That resource says he was paid for every 32 pages. In any case, it seems like for at least some of his early books like The Pickwick Papers, he was paid proport

That resource says he was paid for every 32 pages. In any case, it seems like for at least some of his early books like The Pickwick Papers, he was paid proportional to volume of output and not to sales/quality. (For some of his later works he earned a percentage of sales.) It's a perfectly fine model for people who aren't already as famous as late-career-Dickens and it can nonetheless produce profound work.