Publishing expanded supplementary information as separate paper
Plagiarism is using something without attribution, presenting it as your own. Self-plagiarism is related but a bit different. If you use your own past published work but don't cite the original it is self plagiarism but because it breaks a chain of citations that a future reader might need for context. The older work provides some context, including its own citations that are needed by future scholars. 

But you can also refer to the older work (of your own) and cite it just as you would the work of another author. In quoting, you can also be a bit more expansive, since you won't be breaking copyright rules, assuming that you still hold rights. But you need to quote, not just copy, so that the origin is clear. 

The other question is about publish-ability. That is up to the journal of course, and the new paper will be evaluated as usual based on novelty and relevance. You seem to indicate that you won't have a problem with that, so it is worth a submission.