Add FacetWP facets with code-based ones, with preference for code-based definitions over db ones with same name

functions.php

<?php 
/**
 * Glob all our JSON facets and register them
 * with FacetWP
 */
function my_register_facets( $facets ) {
	$new_facets_raw = glob(dirname(__FILE__) . '/facets/*.json'); // Or however you want to get your facet definitions
	$new_facets = [];
	foreach($new_facets_raw as $facet) {
		$facet = json_decode(file_get_contents($facet), true);
		$new_facets[] = $facet;
	}
	// Important: Put facets at the front of the array so they get added before db-based ones
	return array_merge($new_facets, $facets);
}

add_filter( 'facetwp_facets', 'my_register_facets' );