I'm a simple employee with some management duties, it's small company (20p), I estimate my wage to be on the low end of the local market (french, not in the capital), but the position is quiet unusual so it's difficult to compare.

At the beginning there was only vague goals, not estimates, now 4y later I can tell how much days a functionality will take within a 30% margin error. But it's only based on personal appreciation, tightly coupled to having personally designed the new arch from the start.

The migration strategy that I chose was to hijack the sales activity and pick which new projects should go on the new version. That way we can stay away from replacing years of specific functionalities for picky customers until we have a good idea of how much the business module is ready for prime-time.

I should definitely write a book, there's so much to say - maybe in 2020! :)