write developer debug statements to separate log

You can tell error_log() where to put its output:

error_log( 'my debugging output', 3, '/my/debug/log/location/log.txt' );

It might be more convenient to add your own function:

if ( ! function_exists( 'my_debug' ) ) {

  function my_debug( $x = '' ) {
    // Uses print_r() in case $x isn't a string.
    error_log( print_r( $x, 1 ), 3, '/my/debug/log/location/log.txt' );
  }

}

// ... elsewhere

my_debug( $the_variable_i_want_to_check );