A proto-Angmar in the Second Age

[Zigûr]

Yes. While referring to Lindon, not Arnor, Appendix A is entirely explicit on this point:

But Sauron struck too soon, before his own power was rebuilt, whereas the power of Gil-galad had increased in his absence; and in the Last Alliance that was made against him Sauron was overthrown and the One Ring was taken from him. So ended the Second Age.

It's emblematic of how doubtful Sauron's means were at the end of the Second Age that his enemies were able to invade his territory, rather than the reverse. Some short-lived early victories against newly-founded Gondor notwithstanding, Sauron was weak* at the end of the Second Age, and his enemies were strong. That's why he spent the next three thousand years wearing his enemies down through subterfuge, unconventional weaponry and proxy wars.
*And this is taking into account the remark in Myths Transformed about Morgoth's First Age power compared to Sauron's in the Second.