As of S/4HANA 2502 Public Cloud on Enterprise Projects milestones have moved from just the project to WBS elements. These milestones still cannot generate the billing plan on the Sales Order but that is on the roadmap for Q1 2026.
There is a BPA developed on BTP that runs through an approval when the financial plan is uploaded to the project.https://api.sap.com/package/sap.build.pa.content.projectbudgetapproval/overview