Cataloginventory Stock Item Update fails on save

I’m trying to update Stock Qty for a Magento Product. I either create a new product or update an existing product with the same call. I do it like this:

    if ($product->getQty() !== ($productData['inventory'] ?? 0)) {
        $stockItem = $product->getStockItem();
        $stockItem->assignProduct($product)->setData([
            'qty' => $productData['inventory'] ?? 0,
            'is_in_stock' => 1,
            'use_config_manage_stock' => 1,
            'manage_stock' => 1,
        ]);
        $product->setStockItem($stockItem);
        $product->setStockData([
            'qty' => $productData['inventory'] ?? 0,
            'is_in_stock' => 1,
            'use_config_manage_stock' => 1,
            'manage_stock' => 1,
        ]);
    }

Yet, when I try to issue save() it keeps telling me either:

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '1234567-1' for key 'UNQ_CATALOGINVENTORY_STOCK_ITEM_PRODUCT_ID_STOCK_ID', query was: INSERT INTO `cataloginventory_stock_item` (`product_id`, `stock_id`, `qty`, `is_in_stock`, `low_stock_date`, `manage_stock`, `use_config_manage_stock`, `stock_status_changed_auto`) VALUES (?, ?, ?, ?, ?, ?, ?, ?) at /var/www/html/htdocs/lib/Zend/Db/Statement/Pdo.php on 235

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`magento`.`cataloginventory_stock_item`, CONSTRAINT `FK_CATINV_STOCK_ITEM_PRD_ID_CAT_PRD_ENTT_ENTT_ID` FOREIGN KEY (`product_id`) REFERENCES `catalog_product_entity` (`entity_id`) ON DELETE CA), query was: INSERT INTO `cataloginventory_stock_item` (`qty`, `is_in_stock`, `manage_stock`, `use_config_manage_stock`) VALUES (?, ?, ?, ?) at /var/www/html/htdocs/lib/Zend/Db/Statement/Pdo.php on 23

What am I doing wrong here?