When you are selling only services, there are no need to use a sale order.
Also the invoicing of all those example are often a mix of partial advance payment and invoicing on progress or timesheet.
But this topic remember New invoicing method when selling both goods and services - #3 by ced but as I said I do not think this new task object should be project. project and sale are different workflow of selling.
Indeed I think we are missing a quotation report for the project.
On sale it would be managed by creating multiple sale so as for the project.