About the oil color
https://riautomotive.net/dark-brown-transmission-fluid-needs-to-be-changed-right-away/
Since you can't see the color on the dipstick, you either check by sucking a sample, by wiping the stick on a white cloth or change it regularly. In my experience, that 5000 miles could be a good limit - I think. The oil is used so little that this is not a cost issue.
If you change the oil on time, I think you don't need to flush the entire oil chamber.