Hi there, @NoGoodSoftware.
I'd be glad to join the thread and help check this further for you.
If you don't have the option to to customize the form in QuickBooks Online (QBO), it's possible that you don't have the rights to do it. Only the master administrator has the change to customize the template in QBO.
On the other hand, if you're trying to customize old forms and it syncs to all of the templates, then that is how QBO works.
As of now, you have the option to use your own form styles by exporting custom invoice form styles to QuickBooks. For the detailed instructions, you may check this article: Import custom form styles for invoices or estimates.
As always, don't hesitate to add a post/comment below if you have any other questions about forms and invoices. I'm just a post away to help you!
