Is it self-plagiarism if I am up-front about similarities?

As long as the new paper extends the prior one, presents new data or analysis, and you cite properly, you are fine. It is best to pretend you did not write the prior work and cite accordingly, because you assume no special status even though you authored it. This would only become problematic if you attempt to pass off the same work (even if edited) as a distinct novel work.

It sounds like you are on safe ground. Moreover, if you are merely replicating previous results for context, it might not even be necessary to repeatedly cite the original paper at each step. You could simply cite the original paper up front, explain that you are extending it, and that some results are repeated for context and completeness. So called "self-plagiarism" involves an attempt to "pass off" your old work as new work, so check your paper to see if this is occurring. From what you have described, you should be fine.