So I changed FontName to use file name instead of font’s stored name, cut down glyphs to 2 sections - Basic Latin and Hangul Syllables, and while it took off NullReferenceException issue that I had before, it still gets stuck in building spritefont again. It seems like this issue is neither caused by my .spritefont file, nor TTF file itself.
EDIT: The version is still 3.8.0.600. Nothing big change was done, except the .spritefont resource file.