I agree with you. A Certified & Good SEO agency will provide more professional & top-class services than so-cold fiverr seller.
Here are some tips to find out Disavow toxic, low-quality and spammy backlinks. Such as:
- Firstly check your backlink on ahrefs
- Mark all of the 0 Traffic websites link
- Mark all of the low DA websites
- Mark 18+ backlink & thin content pages
- Mark all of the high spammy score backlink