hello

 1) some apps (extension) propose this feature (not for free) and nicely customizable

2) this is absolutely unclean, but a previous answer to a similar question helped me

            update addons/point_of_sale/static/src/app/screens/payment_screen/payment_screen.js

            locate the function  async validateOrder(isForceValidate), under the first instruction, you may add:

            //---- Update Begin -  Force Invoice to be generated

            if (!this.currentOrder.is_to_invoice()){this.toggleIsToInvoice();}

            //--- Update End

HTH