I've used Semantic UI a bit and recommend it. Your mileage may vary but...

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I feel the framework is updated enough. I suppose this is somewhat open to each developer's interpretation of what "enough" is. The last notable release was July and the last commit to their master branch was at the end of September. As long as there aren't major bugs, and the framework is fairly feature-complete, I don't personally need weekly or monthly updates so this works for me.

I haven't run into any real annoying bugs. Yet.

I only support modern browsers in all my client work unless I am being paid specifically to support IE10 and below. Therefore, their lack of support for older versions of IE9 and below doesn't affect me. It works with all modern browsers.

The docs are your friend regarding components. They address creating components and .less files with theming variables, etc. on their site. If you follow their guidelines your component should blend right in.

It does not "need" a library like React, but right in their documentation it says:

"Semantic UI components are designed to be compatible with libraries that tightly manage UI lifecycle like React. No special bindings are needed."

There are also Vue versions of the Sematic UI components and as mentioned in the quote from their docs above, it's really straight forward to implement Semantic UI with other libraries.

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Semantic UI has really good documentation. Most of the things you've mentioned are covered there so I would probably start there when evaluating if it will meet your needs as a whole.