Hi there, @rem179.
At this time, customizing estimate templates to break up items by heading isn't available in QuickBooks Online. Though, you can consider importing and designing custom form styles for your estimates. However, it depends on the fields you're trying to add.
I can see how beneficial it is for you and your business to customize estimates with multiple headings. But I want to let you know that your voice matters and I'm submitting feedback directly to our product engineers for consideration.
For now, you can visit our blog site so you'll be able to get the latest news about QuickBooks and what our Product Care Team is working on.
Please keep in touch if there's anything else I can do to help you succeed with QuickBooks. I've got your back. Have a great day and be safe.