From 12.2 index monitoring is enabled out-of-the-box. You're on 18c. So to see which queries use your indexes and how, all you need to do is query DBA_INDEX_USAGE.

Read more about this here https://oracle-base.com/articles/12c/index-usage-tracking-12cr2

For the effect on writes, use a procedure like this:

create or replace procedure test_indexes ( num_runs int ) as
  start_time pls_integer ;
  time_taken number ;
begin
  start_time := dbms_utility.get_time ();
  for i in 1 .. num_runs loop
    
    /* YOUR DML STATEMENTS HERE */

  end loop;
  time_taken := ( dbms_utility.get_time () - start_time ) / 100;
  dbms_output.put_line ( 
    'Total duration: ' || time_taken || ' seconds; ' ||
    ' seconds/execution: ' || ( time_taken / num_runs )
  ) ;
end test_indexes;
/

Run this a few times to get average execution times. Then add/remove indexes to see what impact they have:

exec test_indexes ( 10000 );
truncate table t;
exec test_indexes ( 10000 );
truncate table t;
exec test_indexes ( 10000 );

-- DROP/CREATE INDEXES and repeat