Is Syncthing running under the same user account as where you mapped the drive? Sometimes Windows hides mapped network drives when you e.g. run an application as the administrator account.
Enabling SMBv1 is a reaaally bad advice. The analysis by @acolomb is spot on. The network mounts on Windows are tied to the user. A service account won’t be able to use it.
It was offered as a test only, just to see if the version was the cause. The user / password would flag up a logon box and once those credentials were entered and saved, St would use those.
Maybe it’s best to see if windows is able to access the shared files before thinking it’s a St issue?