Hi Nina,
Thank you for considering TSF.
Yes, the Focus extension is comparable to that functionality.
We made it Premium because it’s built on a dictionary API service (the same one Google uses). But, there’s a caveat since Google’s search engine doesn’t support the German dictionary (their “dictionary” supports definitions and similar words, but not synonyms and inflections): https://tsf.fyi/e/focus/#faq/what-if-my-site-s-language-doesn-t-support-synonym-or-inflection-lookup.
There aren’t any practical differences regarding Focus keywords between TSF and Yoast; they’re built on the same principle. However, TSF won’t be constantly screaming at you, and we resolved all editor performance issues.
If you have further questions about our premium services or require support with the extensions, sales, or your account with us, please contact us privately: https://tsf.fyi/contact. Thanks!