One tool we work with that has been built in-house is called the ‘M&A Playbook’. This is used as an M&A management guiding tool, including scenarios, to fit all types of M&A’s, regardless of how advance the user is. It both teaches, explains, and provides tools. Attached within the book, are templates, and historic examples of M&A’s that company has had.
Technically it is a project management tool but I always like the business architecture approach to mapping capabilities, value streams and stakeholders. It clarifies the picture of who does what and surfaces gaps in capabilities and decision making.