Don’t forget you need execute permission for things like listing directory content, so rw is not enough.
Also, make sure that parent directory where .stfolder is being created has rwx for the user running the process.
Lastly, I think you might hit other issues by not handing ownership to syncthing. I don’t have exact failure scenario in mind, but my spider senses say it might not work.
Docker et al, support user remapping, so I don’t see why you insist on syncthing not owning the files.