I've recently started helping with my father-in-law with his B2B (industrial manufacturing) site. The site has a complicated history of poor maintenance/management. I discovered that the site has 1.6 million pages (8.86k indexed, 1.58m not indexed). But the actual site has only 150-200 "real pages". The rest are likely legacy pages from earlier builds and there was a period where the site experienced a "Japanese keyword/doorway hack" (which was eventually cleaned up).

Is there a way to completely remove the pages so Google doesn't even crawl them? I've also done some 410 redirects, but it doesn't seem to make any difference.

Some pages have ranked in the top 10, but newer blog content pages are not ranking.

The site's domain authority is very low, but the company wants to keep the domain. Is there hope for this site? Would engaging an SEO agency help?