If you use the Builder to create the tabs and custom fields, you can drag and drop to re-order the tabs and fields.
If you are using the code to register the tabs and custom fields, just move a tab above another to re-order them.
Unfortunately, the Builder does not support re-ordering the tabs on the settings page. You can use the tab field in the Field Group to re-order them. I will inform the development team to consider supporting this option in future updates.
Yes, you can use the builder to export field groups to JSON files, edit the file to add custom fields, and re-import to the builder. But please be careful with this way, it will break your field group if there is a small error like a typo.