I believe this is doable. However, I don’t think that WooCommerce stores product stock in order meta – we need to use product meta instead. So in your case, we need to add a PHP snippet to your site. If that’s ok – there is the
alg_wc_order_status_rules_do_apply_rule filter in our plugin. It allows you to customize the conditions of when the rule should be applied. So you need to add a simple rule, e.g., Rule #1 / Status from: Processing / Status to: Completed. And finally, add this snippet to your site (“Example #3: Check if the order has any products with stock quantity lower than or equal to 5”).
Please give it a try and let me know what you think.