
Hello Roman,
Thank you for sharing on the community forum.
The first option is not possible to change the form as it is locked for the create order, you can either recreate the functionality and use another button for the action that you want to achieve.
If this is complicated, my opinion will be to either use a Microsoft flow where you will create a another BPU record with all the information and the customer will have to go to the form and edit them.
I would suggest you to try to do one of the 3 solution that you have described to see what fit to your business requirements.
If you any issue by doing it , may be you can share your experience and what is blocking you:
1. Use of Power Automate (Flow), with the existing button "Create an order":
If: 'Type of quote' = BPU
Then: Creation of the order (standard) + Creation of the BPU + Deletion of the order
--> Requires going to the BPU to complete the fields
2. I have an existing process that allows all quotes related to an opportunity to be closed if one of them is ordered. In this case, this process would create a BPU for quotes with the field 'Type of quote' = BPU
--> Requires going to the BPU to complete the fields
3. Do not use the Quote entity for BPUs sent to customers and not validated, but use the BPU entity from the start, with a 'Draft' status for example. When the client validates the BPU, change the BPU to 'Active'.