services.tahoe.nodes.<name>.web.port

The port on which the Web server will listen.

This is the correct setting to tweak if you want Tahoe's WUI to listen on a different port.

Type
16 bit unsigned integer; between 0 and 65535 (both inclusive)
Default
3456
Declared
<nixpkgs/nixos/modules/services/network-filesystems/tahoe.nix>