in several forums, users criticises the Tread app and wouldn't no longer use them.
You can go the way, @guzt describes, but there is no guarantee, the route presented in your Garmin after import is identical with your planned route on PC. Because every navigation device/app is interpreting imported data by their own algorithms!
To set enough route points (via or shaping) in planning, could prevent different interpretation after transmitting to other navigation devices/app.
I did this a long time - but was not satisfied and tired about.
In the end, I found an easy and perfect way: do the planning in MRA and use for navigation the MyRoute Navigation. There is no need of file transmitting anymore, because of automated synchronisation.
Try it, you will love it...