Demonstrate reasons for change in a language that Finance understands
The deck in the first link shows a conceptual roadmap that could be used as guideline. As organization structures or maturity levels are not the same, the startpoint could for every company be different. Important from a business and finance perspective is that the financial reports are read and studied and when available investors calls that you listen in and analyze to get a grip of the company compliance statements, business objectives & strategy and short and long term forecasting, etc. Those are the sales arguments for change you need.
Based on that define the areas where indirect tax (or TP) has the biggest impact and lowest performing process or technology.
This is further explained in 'Achieving stakeholder satisfaction' and 'Writing a business case, problem statement and calculate Return on Investment'.