async appears to not use multiple threads C++

Question

my async calls appear to run in serial execution .. leaving me to believe Im not doing something correctly in the code or.. perhaps not understanding something

At a high level I take in 100,000 or so strings, use them to perform a query rate limited to 1 per second per connection.. I am starting with 4 connections to the service and want to use all 4 concurrently .. wait for all their answers to come back and then combine the results

Main.cpp

  
  struct DataFetcher {
    DataFetcher(cpool::ConnectionPool& tws_conn_pool): conn_pool(&conn_pool){
      std::cout << "DataFetcher() this->conn_pool == "<< this->conn_pool << std::endl;
    }

    std::vector<std::string> process_data(std::vector<std::string> sub_batch_vector){

      std::for_each(sub_batch_vector.begin(), sub_batch_vector.end(),[](std::string query_str){
        std::cout << query_str << ";";
        std::this_thread::sleep_for (std::chrono::seconds(1));
      });

      cpool::ConnectionPool::ConnectionProxy proxy_conn = this->my_conn_pool->get_connection();

      proxy_conn->is_healthy();

      // will do more stuff later here

      return sub_batch_vector;
    }

    cpool::ConnectionPool * const tws_conn_pool;
  };

What is the proper way to use this async call within a loop.. there will be a variable number of ServiceConnections available so I'd like to do something like this

std::vector<std::future<std::vector<std::string>>> results_vector;

for (int i=0 ; i < this->tws_conn_pool->size()-1; i++) {
      
  std::vector<std::string> subvector = {queries_list.begin() + (batch_size*i), queries_list.begin() + (batch_size*(i+1))};

  DataFetcher fetcher = DataFetcher(*this->conn_pool);

  auto fut = std::async(std::launch::async, &DataFetcher::process_data, &fetcher, subvector);
      
  results_vector.push_back(fut);
}

but this seems very obtuse and doesn't actually compile..

 from /Server/cpp_server/src/_server.cpp:1:
/usr/include/c++/9/ext/new_allocator.h: In instantiation of ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::future<std::vector<std::__cxx11::basic_string<char> > >; _Args = {const std::future<std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >&}; _Tp = std::future<std::vector<std::__cxx11::basic_string<char> > >]’:
/usr/include/c++/9/bits/alloc_traits.h:482:2:   required from ‘static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::future<std::vector<std::__cxx11::basic_string<char> > >; _Args = {const std::future<std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >&}; _Tp = std::future<std::vector<std::__cxx11::basic_string<char> > >; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::future<std::vector<std::__cxx11::basic_string<char> > > >]’
/usr/include/c++/9/bits/stl_vector.h:1189:30:   required from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::future<std::vector<std::__cxx11::basic_string<char> > >; _Alloc = std::allocator<std::future<std::vector<std::__cxx11::basic_string<char> > > >; std::vector<_Tp, _Alloc>::value_type = std::future<std::vector<std::__cxx11::basic_string<char> > >]’
/Server/cpp_server/src/_server.cpp:175:35:   required from here
/usr/include/c++/9/ext/new_allocator.h:145:20: error: use of deleted function ‘std::future<_Res>::future(const std::future<_Res>&) [with _Res = std::vector<std::__cxx11::basic_string<char> >]’
  145 |  noexcept(noexcept(::new((void *)__p)
      |                    ^~~~~~~~~~~~~~~~~~
  146 |        _Up(std::forward<_Args>(__args)...)))
      |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /Server/cpp_server/src/_server.cpp:4:
/usr/include/c++/9/future:782:7: note: declared here
  782 |       future(const future&) = delete;
      |       ^~~~~~
make[2]: *** [CMakeFiles/_server.dir/build.make:63: CMakeFiles/_server.dir/src/_server.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:76: CMakeFiles/_server.dir/all] Error 2