Use a Custom Field as a buoy to supersede SearchWP's relevance weight sorting

searchwp-customizations.php

<?php

// Use a Custom Field as a buoy to supersede SearchWP's relevance weight sorting.
// @link https://searchwp.com/documentation/knowledge-base/custom-field-prioritize-results/
add_filter( 'searchwp\query\mods', function( $mods, $query ) {
	global $wpdb;

	$meta_key = 'my_buoy_meta_key';

	// Add the buoy to these post types:
	$post_types = [ 'post', 'page', ];

	foreach ( $post_types as $post_type ) {
		$mod        = new \SearchWP\Mod();
		$alias      = \SearchWP::$index->get_alias();
		$meta_alias = 'my_searchwp_sort_' . $post_type;

		$mod->column_as( $wpdb->prepare( "(
			SELECT meta_value
			FROM {$wpdb->postmeta}
			WHERE
				{$wpdb->postmeta}.post_id = {$alias}.id
				AND {$wpdb->postmeta}.meta_key = %s
			)", $meta_key ),
			$meta_alias );
		$mod->order_by( "{$meta_alias} + 0", 'ASC', 2 );

		$mods[] = $mod;
	}

	return $mods;
}, 30, 2 );