Deadlift technique generally good. Yeah, a bit of flexion around T12-L1 but nothing I’d cry about.
Good mornings are supposed to be a relatively controlled movement, primarily based on flexion about an axis through your hip joint. This way the hamstrings will eccentrically work to control the posterior pelvic tilt as you flex forward. On the way back up, your lumbar extensors will work concentrically to bring you back to vertical. What you are doing is too similar to a squat.
I would look to substantially reduce the knee flexion which you are using throughout the movement, and focus on imagining a stick through your hip joints that you are pivoting around. As suggested above, push your bum back a bit before initiating the downward movement and maintain a neutral spine throughout. Very partial knee flexion should be used as a brace for the movement, it shouldn’t be the exercise itself.
You will probably have to reduce the absolute load, but don’t worry, doing it like this will get your hammies and low back more
Looking good and strong though!! Good stuff.