Today I found four volumes on the history of Balassagyarmat, written in Hungarian, online at hungaricana.hu:
https://library.hungaricana.hu/en/collection/mltk_megy_nogr_nite/
The 1995 volume in particular has details on the Jewish history, population, and the holocaust, including two lists of martyrs (480 and 290 names, possibly with some overlap). See here: https://library.hungaricana.hu/en/view/NOGM_NITE_02/?pg=0&layout=s
The text is searchable, and you can copy/paste into Google Translate, which is currently doing a tolerable job of Hungarian -> English translation.