FWIW, I just checked and the only permission I have granted to SyncThingFork is "notifications".

I think it optimistically requests all, but I can testify that it works fine without granting it all storage access. I just had to create/select a subdir in my SyncThingFork directory.

I don't really know how the restricted storage access works. Maybe it's that if /another/ app has already claimed a directory, that SyncThing needs more permissions to be able to sync it? But that seems unlikely, just speculating.