Not sure where you read about PayPal using predefined values but if you are using OpenCart 4, I'm assuming that you are referring to the PayPal Checkout Integration payment extension. For PayPal Checkout Integration payment extension, there's literally an "Order Status" option page that allows you to assign your OpenCart order statuses to PayPal's payment statuses.
Anyway typically, an OpenCart payment extension should allow you to assign order statuses to the payment statuses. If it doesn't allow you to that, then it's probably a poorly-built extension and you probably would have to:
1. Look for alternatives
2. Make adjustments/customise it
But in general, as long as your are using official payment extensions and/or payment extensions developed by reputable sellers, you shouldn't have any issues with having custom order statuses. Almost all my clients have their own defined order statuses and they don't have any issues.
Do also note that if you try to delete an order status that is used by one or more orders, you won't be able to delete it.
Anyway typically, an OpenCart payment extension should allow you to assign order statuses to the payment statuses. If it doesn't allow you to that, then it's probably a poorly-built extension and you probably would have to:
1. Look for alternatives
2. Make adjustments/customise it
But in general, as long as your are using official payment extensions and/or payment extensions developed by reputable sellers, you shouldn't have any issues with having custom order statuses. Almost all my clients have their own defined order statuses and they don't have any issues.
Do also note that if you try to delete an order status that is used by one or more orders, you won't be able to delete it.