Hi FlyCat, were you able to find a way to have the watch remain on the countdown timer screen? I'm looking for this from a different perspective , trying to keep track of the remaining time as a referee for a field hockey match whilst recording an activity in the background.
Controls time out after not very long. This is an issue for me using the countdown timer alone, without first starting an activity. I don't think there's any way around it. See also: https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-735xt/163715/keep-watch-from-closing-widget-music-control
You may want to suggest to Garmin that they add a countdown timer to the Yoga app (which I've never used). The Strength app has built-in (stopwatch) timers for sets and rest periods.
It seems that the Yoga app of the new Vivoactive 4 series includes a timer. You can also create a workout in Garmin Connect and send it to the device. See Ray Maker's review.