I believe that you’re not correct and that Mobian and PostmarketOS do support FDE.
- PostmarketOS. See pmbootstrap install --fde . See this thread Running PostmarketOS on My Librem 5 - #9 by fossman
- It’s my understanding that librem5-flash-image is the flash+installer script and this installer sets up LUKS FDE. InstallingDebianOn/Purism/Librem5Phone - Debian Wiki