To make the module even more flexible, it would be great to have checkout flow panes per field / field group, instead of a large one.
You may otherwise run into the issue, that you need to split fields around the existing panes. One example could be the need to add a "Notes" textarea and an "Accept terms and conditions" checkbox.
While the checkbox will typically the last item before the submit button, the notes should often appear in the middle or top of the page before the order items. This isn't possible currently.
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Another method would be to allow having even more viewmodes, but I think that wouldn't be that flexible.
A third one might be to have a dedicated field group type just for this and if you need separate panes, you wrap the fields into these. But that might be the most complex one ;)
I think the basic idea is the best and most simple.