I would suggest looking at @theendofcake 's String review page!
Personally I think the C3 Pro string is just good universal string for its price. Sochi also produces good product. I personally use Yoyo String Labs Nytro or KaPOW! for when I want a more refined string feel.
The aforementioned page:
Also on your thing about thick string- it’s all about preference and gap width. Narrow-gapped yoyos get snagged on thick string more easily and thus may bind unintentionally. However, they tend to be more comfortable than narrow string, last longer (per my experience- it could also just be my perception) and do just fine when paired with a wide-gap yoyo.
Thin string to me is gross but it is important to use in 2A. It also is compatible with more yoyos as they don’t have as much of a presence in the gap and are far less likely to unintentionally trigger the bind response.