The one catch with this new suggestion is that it expects symbol keys for the inner params values - which is not the case for the form data I’m dealing with. Yes, I could wrangle the data further… 
puts outer_schema.(foo: { path: "something" }).inspect
# #<Dry::Schema::Result{:foo=>{:path=>"something"}} errors={}>
puts outer_schema.(foo: {"path" => "something" }).inspect
# #<Dry::Schema::Result{:foo=>{"path"=>"something"}} errors={:foo=>{:path=>["is missing"]}}>