I too run a crawler that visits a lot of pages, although not at a particular high frequency. We visit hundreds of sites and each site then has a custom bot that essentially has two methods: find_links and extract. The first finds more links to visit on the site (e.g. navigates and follows pagination) whereas the latter finds and stores records. Is this similar to your approach?

Incidentally, at scale I find that the more tricky part is the whole orchestration (Schedule crawls, make sure resources are used most efficiently without overloading the target sites, properly detecting errors) is the hardest part.