Hello, bugsbegone.
Thanks for sharing the steps you've done so far to show the custom fields on the estimate. I can provide you with some information and help you in resolving the issue.
Due to a recent update under the new invoicing experience, the custom field feature is now available in QuickBooks Online Plus and Advanced subscriptions. With the new estimates and invoices, some functionalities aren't available with the Essentials subscription.
You can switch to the old version so you can use the custom field again. I'll show you how.
- Click the + New icon.
- Select Estimate.
- At the upper right side of the page, hit Old Layout.

- This should bring you to the old version wherein you can see the custom field you've created.
For your reference, feel free to visit this article for more details: See what’s new with estimates and invoices in QuickBooks Online.
In addition, I've included this resource in case you want to personalize your sales forms so that they only display the information that's relevant to your company: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Let me know if you need further assistance with the custom field feature. We're always here to help you.