This v1.16.1-2 release adds a hidden feature to pass additional commandline arguments to the syncthing executable.
You need to write these using the application defaults configuration from the Terminal.app.
The only current limitation the parameters cannot contain spaces!.
In the example below the audit log is enabled: defaults write com.github.xor-gate.syncthing-macosx Arguments '--audit --auditfile=/Users/JohnDoe/staudit.log'
See request #154