The most common technical SEO mistake is treating the whole site like it deserves equal attention. It usually does not. Some pages matter for search, some matter for conversion, some matter for credibility, and some are just there because the site needs them. If you try to push all of them equally, the site gets noisy fast.
I usually start by listing the pages that would still matter if rankings disappeared tomorrow. Guides that solve a real question. Project or service pages that help the right person decide. Reference pages that stay useful over time. Those are the pages worth supporting technically.
This sounds obvious, but it changes the whole workflow. Once you know which pages matter, internal links, crawl paths, titles, and schema become support work instead of busywork.