gocryptfs (https://nuetzlich.net/gocryptfs/) has a reverse mode (mounts folder as a virtual encrypted folder) which can then be added to Syncthing thereby adding desired encrypted backup functionality.
Works perfectly right now, is audited (https://defuse.ca/audits/gocryptfs.htm) albeit not the reverse mode, fixes security issues and file name limits of EncFS, and can be used on Windows (https://github.com/bailey27/cppcryptfs/releases) through a GUI.
Command to create encrypted virtual directory:
gocryptfs -init -reverse "Directory to be encrypted" && mkdir "Virtual directory"
Mount encrypted virtual directory (recommend leaving spaces or special characters out when using echo to send password):
echo PASSWORD | gocryptfs -reverse "Directory to be encrypted" "Virtual directory"
or prompt for password:
gocryptfs -reverse "Directory to be encrypted" "Virtual directory"
Then add virtual directory to Syncthing. If need to unmount virtual encrypted directory:
sudo umount "Virtual directory"
gocryptfs is also written in go. Rather than coming up with a new solution this probably/maybe could be integrated into Syncthing if really desired.