Have a PT check while you perform these exercises. It's possible your lower traps are so weak they aren't being activated, even when doing lower trap exercises. This can occur if they are weak or you have other muscles taking over. I've had a PT that made me perform Y raises on a flat bench, but one arm at a time, and she had me bend my elbow at 90 degrees to really focus on the trap.
My point being is to start with some low trap activation exercises and just feel the muscle first. The picture you showed is similar to some activation mobility exercises I have seen(notice the elbow bent at 90 degrees?).
Some tips are to try to depress your shoulders(This is harder than it seems through out an exercise), start with no weight, retract your shoulders, and focus on your lower traps doing the work, with high reps. after a while, you should feel a burn in your back, whereas some feel it incorrectly in upper traps or shoulders.
You can also try other exercises. Here are some that have worked for me:
- behind the back straight bar reverse shrug(so have the bar behind you, and opposite of a shrug exercise, instead of shrugging your shoulders up, depress them in order to move the bar down.). This exercise can be a bit tricky to load up but its good to focus on depressing shoulders.
- lat pulldown using only shoulder depression (you're not pulling down the lat bar, only shrugging the weight down by depressing your shoulders)
- Kelso Shrug - set your body on an incline bench with a suppinated grip on a barbell and depress shoulders, retract them, and "shrug" the weight to the bench, almost act like you are trying to break it apart.
- Y raises (one armed variations might work better for you, start with no weight)
- I raises, straight arm rope pulldowns, facepulls, dumbbell rows with neutral grip work your rhomboids and a bit of your lower traps.
- dips - focus on depressing your shoulders. If you're doing chest dips, make sure to keep shoulders depressed. the trap version will have you solely depressing your shoulders.
From my research, once you learn how to activate them, there aren't a lot of "low trap" isolation exercises, but the gold standard is Y raises, as it depresses the shoulders and rotates them