@PeterB
Linux Mint Debian Edition (LMDE 6) seems to do the trick. That version of Linux Mint actually has qt6ct in the software channel, although an older version, 0.7-3. I installed your newer version, 0.9-2, and it seems to work fine for the most part.
Interestingly, it would not allow me to set the font as Ubuntu Regular; instead insisting on Ubuntu Light Italic. It allowed me to use Noto Sans, which is close enough. But any insight on that font issue would be appreciated.
Thanks again for helping me out with this!