Yes, you can add a subtotal in an estimate, @Penny923.
We use this as a line item in QuickBooks Online to total a group of products, discounts, and services amount. Let me share with you the steps and a specific screenshot that shows an item combined with a discount which then I included a Subtotal:
However, since you're looking at the different side of a subtotal where it doesn't get added to the final amount, I recommend sending feedback. You can also include your suggestion about adding descriptions about a specific amount covered on a subtotal.
This way, we can boost your QuickBooks experience once by chance, the feature you're suggesting for, is added in the future updates.
You may want to personalize your sales forms by adding more details and fields, take a look at this article for more information: Customize your sales forms in QuickBooks Online.
Let me know if this helps and if you have any other questions. I'm here to lend a helping hand. Have a splendid weekend!