Something must change either in the process or the volume. Moving your high volume vendors to the AN for PO flip or non-PO invoicing directly into Ariba would reduce the number of steps involved for that invoice. Are there any steps that could be eliminated? Could you triage your invoices, say invoices under a certain $ from certain suppliers don't need to be signed or are assigned coding. Why are you printing invoices and then scanning? What is the value of bar coding, why not attach the invoice to the Ariba invoice? You can also look at an outsourced scanning vendor to at least get you an image. Moving to a PO for purchases should eliminate the need to code and sign the invoice
I don't understand the business value to several of your steps, they seem redundant and there may be alternative processes that simplify your life yet provide your company with the desired level of control and record keeping.
Of course, if your company is unwilling to acknowledge the workload by either resourcing or efficiency gains, that shouldn't be your problem. Your skill set is very mobile.