How to manage around 90k 301 redirects after migration from old CMS

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Here is a trick, redirections in WP are complex. You can get 3 situations when they can happen.

  1. There is matched to your url rule, but no posts returned on wp_query from generated query_string.
  2. There is no rule that match your 404.
  3. Other Situations ( you wouldn’t run into this on your own, so don’t worry. )

Based on thi, 404 redirection can be triggered on 3 wp points.

  1. request
  2. wp_headers
  3. template_redirect

So you can use this as template for your redirection logic

add_filter( 'request', function( Wp_Query $wp_query ){
    if ( isset( $wp_query['error'] ) && 404 === intval( $wp_query['error'] ) ){
        // redirection logickgoes here...
    }
    return $wp_query;
});

add_filter( 'wp_headers', function( $headers, $wp ){
    if ( isset( $wp->query_vars['error'] ) && 404 === intval( $wp->query_vars['error'] ) ) {
        // redirection logic goes here...
    }
    return $headers;
}, 10, 2);

add_action( 'template_redirect', function( $params ){
    if ( is_404() ) {
        // redirection logic goes here.
    }  
});

These are a bit hacky ways to do redirection on 404 error, but this is what I come up when run into sites migration myself.