Dear Philipp,

I do not have the reference to Wezler, and I do not know about the particular case
of the Sāṅkhyakārikā, but it is quite clear from the Tibetan transcriptions of the Sanskrit
titles of works in which kārikā is used that this usage is not "modern". Here are the instances
from the Tibetan canons, as retrieved by searching the University of Vienna's useful site on
Kanjur studies:
https://www.istb.univie.ac.at/kanjur/xml3/xml/index.php