Lou,
"I just wanted to throw a few factors out there in order to encourage a response like yours - packed full of useful next steps for me to evalaute this further."
THAT is priceless 
Pagination:
Loading all content on one page and using a "more" button to "reveal" it, is not a best practice. Individual pages need to exist for individual sub-topic based content. This is especially true since it now appears that Google, while indexing content initially hidden to users, is likely giving less value to that hidden content than content immediately seen.
Pagination is important IF it is executed properly. If you have tens of thousands of results in paginated lists, is that one paginated group, or are they split out into separate groups based on similarity of content? If it's all just one massive group, that's likely another problem to look into, since pagination is meant to be used to say "these pages all contain links to other content where the entire group comprises very similar content around one primary topic".
Internal linking should always point more to main category page destinations than individual pieces of content. It would be unnatural from a usability perspective to link more to individual pieces of content, and thus it would be bad for SEO.
5,000 or so average crawl errors - what is causing those? Are they 404s? Were they previously valid pages? If so, those typically need to not generate 404 but instead be a direct 301 to a highly relevant live page (and where internal links within the site are updated accordingly).
So many more issues to consider...