Now that copy-uploading from Commons to Wikisource is whitelisted (T294824), PWB can directly perform the copy-upload.
However, this fails with
File "/home/user/src/pywikibot/pywikibot/specialbots/_upload.py", line 414, in upload_file
success = imagepage.upload(file_url,
File "/home/user/src/pywikibot/pywikibot/page/__init__.py", line 2410, in upload
return self.site.upload(self, source_filename=filename, source_url=url,
File "/home/user/src/pywikibot/pywikibot/site/_decorators.py", line 92, in callee
return fn(self, *args, **kwargs)
File "/home/user/src/pywikibot/pywikibot/site/_apisite.py", line 2882, in upload
return self.upload(
File "/home/user/src/pywikibot/pywikibot/site/_decorators.py", line 92, in callee
return fn(self, *args, **kwargs)
File "/home/user/src/pywikibot/pywikibot/site/_apisite.py", line 2610, in upload
raise ValueError('Can request stash information only when '
ValueError: Can request stash information only when using a file name.This appears to be because the file_key/offset should only be set when there's a filename, not just a file URL.