This kind person had the solution here: viewtopic.php?t=8173#p28453
So I set up Wifi FTP Server - https://play.google.com/store/apps/details?id=com.medhaapps.wififtpserver - and used these settings in FreeFileSync:

It seemed to work fine and I was very happy.
But when I clicked Compare again, FFS wants to create a ton of files on the right side (my phone) that really already exist on the phone (FFS just synced them itself a couple of minutes ago).
I wondered what would happen if I synced anyway, so I right-clicked just one file on the left and clicked Synchronise selection. What happened was another file was added alongside the original file on my phone with (1) appended to the file name.
This is obviously a bit of a disaster.
Am I doing something wrong? How can I stop this happening? Why isn't FFS seeing the files it just created (they're all images, by the way).
Thank you.
Windows 11 Home
Samsung S9