For variants that you want to allow pre-orders, make sure that the “Track quantity” option is enabled, even if the variant is currently out of stock.
Theme Customization:
You can connect with any Shopify developer and modify your Shopify theme to display a pre-order option for out-of-stock variants. This may involve customizing the product page template and the cart page to add a checkbox or another form element indicating the customer’s intent to pre-order.