The diagonal is attached too close to the corner. Move it several inches to driver's right to give yourself more head room. You'll also want to wrap the whole head area with high-density foam, so you need room for that plus a helmet, and still be able to reach the headrest.
And as someone else said, you need the horizontal bar behind the seat to attach the harnesses, but also support the seat. That is a curved bar from the left vertical to the diagonal. And then from there to the right vertical a straight bar.
I can't tell from the pictures, but the attachments at the floor should be boxed. Tubes pushing through the floor when the car is upside-down is a common failure.
There's usually an additional diagonal bar from top-right to left-lower rear mount-point.
And then horizontal bars going straight back from the vertical bars to the rear mount points
You should be using 1.75” x 0.095” DOM, MIG welded.
Read the rules for inspiration:
SCCA
NASA
PCA