What are the most effective steps to bring the Spam Score down to below 10%, or at least under 30%?
get better links and / or disavow the ones with high spam score
On average, how long does it take to see noticeable improvements after making SEO or structural changes?
that's anyone's guess. I have personally never done such tests because it's silly as google has 1000s of rules and exceptions for every niche, every keyword, every site... so it's basically impossible to gather averages across all niches and keywords.
And "noticeable" can mean anything depending on your goals, expectations and circumstances (niche, SEO you've done to the site, competition strength, etc). So, I don't think anybody can answer this either...
Tools like WebsiteSEOChecker, DApaChecker, SmallSEOTools, etc.
never heard of them, so probably they're irrelevant...
how exactly do they measure or calculate “Spam Score”?
SS is a MOZ thing, so they're probably pulling it from MOZ.
Are their results reliable compared to Moz or Ahrefs?
I would say no, but I'm not sure what type of results you are referring to, so please explain!
Also, probably no non-premium tool (and not even MOZ actually) can compare to Ahrefs, Semrush, etc, so I wouldn't trust what these sites tell you.
Any advice or recommendations would be greatly appreciated.
don't give a shit about spam score! I have seen sites with 100% spam score ranking #1 and raking in millions of visitors a month (probably banking hard too because of that traffic). SS is just a meaningless abomination invented by MOZ that has no impact on google rankings.
If you want to do SEO correctly just buy a Semrush account (you can find GURU accounts in BHW marketplace for $10-20 a month) and use that to perform all of your SEO stuff:
https://www.blackhatworld.com/forums/account-selling-renting-services.310/
I've never had sites in your niche, but from what I've heard strong backlinks and good keyword research are extremely important for that niche to work, so you might want to focus on those 2...
