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.
QUOTE: “So if you have different parts of your website and they’re on different subdomains that’s that’s perfectly fine that’s totally up to you and the way people link across these different subdomains is really up to you I guess one of the tricky aspects there is that we try to figure out what belongs to a website and to treat that more as a single website and sometimes things on separate subdomains are like a single website and sometimes they’re more like separate websites for example on on blogger all of the subdomains are essentially completely separate websites they’re not related to each other on the other hand other websites might have different subdomains and they just use them for different parts of the same thing so maybe for different country versions maybe for different language versions all of that is completely normal.” John Mueller 2017
Where the free Google tools can provide complementary value is in fact-checking. If you're checking out more than one of these SEO tools, you'll quickly realize this isn't an exact science. If you were to look at the PA, DA, and keyword difficulty scores across KWFinder.com, Moz, SpyFu, SEMrush, Ahrefs, AWR Cloud, and Searchmetrics for the same set of keywords, you might get different numbers across each metric separated by anywhere from a few points to dozens. If your business is unsure about an optimization campaign on a particular keyword, you can cross-check with data straight from a free AdWords account and Search Console. Another trick: Enable Incognito mode in your browser along with an extension like the free Moz Toolbar and you can run case-by-case searches on specific keywords to get an organic look at your target search results page.
QUOTE: “Keyword Stuffed” Main Content Pages may be created to lure search engines and users by repeating keywords over and over again, sometimes in unnatural and unhelpful ways. Such pages are created using words likely to be contained in queries issued by users. Keyword stuffing can range from mildly annoying to users, to complete gibberish. Pages created with the intent of luring search engines and users, rather than providing meaningful MC to help users, should be rated Lowest.” Google Search Quality Evaluator Guidelines 2017