Your ability to hold power without constantly looking at your Garmin will definitely get better with time. I have definitely gotten better at it throughout the spring/summer of doing tons of outdoor intervals. I practiced by consciously increasing the time between checking my Garmin. So I wasn’t just going minutes and minutes without looking and suddenly I’m 20W below target. But rather I went from checking every 5 sec to every 10 to every 20 and so on until I was better at going by feel between checking.
Also, when Garmin updated their firmware for the x30 series they got rid of the target power field that was previously recommended. You should use the “Target” data field within the “Workouts” folder. The update is detailed a little more in this post.