Hello swalker-bakertil,
Welcome to Community! I can understand the benefit of being able to import one budget with all classes. I would be glad to provide some insight here!
Recent enhancements were made to Budgeting in QuickBooks Online Plus and Advanced. When you create a new budget you'll have the option to Subdivide by Class, along with the option to Add subdivided budget for, all or specific class types. Follow this link to learn more. In addition, you can also import a budget, but you'll want to download the budget template, to ensure your data is uploaded accurately.
If you would like to see additional enhancements made to the Budgeting feature, please don't hesitate to submit a suggestion to the Development Team. Customer recommendations are greatly appreciated in assisting with improving your overall customer experience in QuickBooks Online.
Feel free to reach back out, if you have any other questions. We would be happy to assist!