Both gallery parameters {$PHOTOS_PATH_RELATIVE} and {$PHOTOS_URL_RELATIVE} give the same output:

com_form2content/p4/f386/gallery93/ (example). Is this correct? Why have two parameters with the same output?

Other than the SIG documentation suggests, the trailing backslash in the output is no problem, so you can use either of the two parameters.

The Simple Image Gallery plugin needs the relative path from the root folder for image galleries. In my case I had to change the root folder into 'images/stories'. Again I'm not sure if the stories folder was initiated by f2c or by me.

So finally no real issue, but it took me some time to figure it all out...

Joomla! 3.9.18
Form2Content Pro version 6.17.4