Originally I was disappointed that Duplicati has SFTP but not FTPS listed. But it appears that FTPS is an advanced option for either of the two FTP options. Making it more directly visible as a protocol may help.
FTP over TLS and FTPS using Explicit TLS howto (Server) may help Filezilla users who want to try it out.
For SFTP, various ones are mentioned in the forum. Bitvise has a How-To, but I think some license limits.
While I find home Windows shares to be hard and unpredictable (I wonder if enterprises have it easier?), sometimes I can make them do what I want. For Duplicati, give UI a UNC path, Username, and Password. Duplicati for this test is running as a Windows Service as default SYSTEM user. This backup worked OK.
Microsoft says that Drive letters are not global to the system, and Network Neighborhood is fading away, therefore Duplicati is probably limited in what it can do. It looks like it adds shares, no NET USE needed.