Definitely use gpx 1.1 with the Zumo XT2 (at least for now). Tread/Zumo XT will not read the Track sent over in the 1.2 format, so all of your shaping points will be lost and your route will just be shortest distance between Via points.
No need to use an SD card - and why would you in the 21st century?
Just export your route in 1.1 from your phone/tablet to Garmin Tread, and it will sync wirelessly with the XT2.
This way, you do not need to add more Via points. I don't know what the limit of shaping points is, but I have not found it yet.