The way I am doing a Conditional Section Menu is by displaying Hugo Menu Items based on a conditional check of the Menu Item .URL vs the Page .Type (Note: .Type is an alias of .Section).
So provided that you have configured a standard Hugo Menu for your Section and Nested Sections called .Menus.section (look into the Docs for Menu configuration -I’m not going into the specifics-)
You can do the following:
{{ range .Site.Menus.section }}
{{ if (strings.Contains .URL $.Type) }}
<a href="{{ .URL }}">{{ .Name }}</a>
{{ end }}
{{ end }}