QUOTE: “Consider where user-generated content might appear on your site or app, and what risks to your site or app’s reputation might occur from malicious user-generated content. Ensure that you mitigate those risks before enabling user-generated content to appear.Set aside some time to regularly review your top pages with user-generated content. Make sure that what you see complies with all our programme policies.” Google Adsense Policies, 2018
All sites have a home or "root" page, which is usually the most frequented page on the site and the starting place of navigation for many visitors. Unless your site has only a handful of pages, you should think about how visitors will go from a general page (your root page) to a page containing more specific content. Do you have enough pages around a specific topic area that it would make sense to create a page describing these related pages (for example, root page -> related topic listing -> specific topic)? Do you have hundreds of different products that need to be classified under multiple category and subcategory pages?
QUOTE: “So it’s not something where we’d say, if your website was previously affected, then it will always be affected. Or if it wasn’t previously affected, it will never be affected.… sometimes we do change the criteria…. category pages…. (I) wouldn’t see that as something where Panda would say, this looks bad.… Ask them the questions from the Panda blog post….. usability, you need to work on.“ John Mueller, Google.
QUOTE: “… it also includes things like the comments, includes the things like the unique and original content that you’re putting out on your site that is being added through user-generated content, all of that as well. So while I don’t really know exactly what our algorithms are looking at specifically with regards to your website, it’s something where sometimes you go through the articles and say well there is some useful information in this article that you’re sharing here, but there’s just lots of other stuff happening on the bottom of these blog posts. When our algorithms look at these pages, in an aggregated way across the whole page, then that’s something where they might say well, this is a lot of content that is unique to this page, but it’s not really high quality content that we want to promote in a very visible way. That’s something where I could imagine that maybe there’s something you could do, otherwise it’s really tricky I guess to look at specific changes you can do when it comes to our quality algorithms.” John Mueller, Google 2016
Ask for a technical and search audit for your site to learn what they think needs to be done, why, and what the expected outcome should be. You'll probably have to pay for this. You will probably have to give them read-only access to your site on Search Console. (At this stage, don't grant them write access.) Your prospective SEO should be able to give you realistic estimates of improvement, and an estimate of the work involved. If they guarantee you that their changes will give you first place in search results, find someone else.