Nice!My car (Seat Leon mk3 ST) was at the dealer last week, so I took the chance to take some pictures from the underside.
I colorized the parts I added to the car:
Red: front suspension cover
Green: front exhaust cover
Pink: rear exhaust cover
Blue: torsion bar covers from Golf 8
I think I've installed all available covers for my car, except for the subframe cover and the jackpoint covers.
View attachment 262587
Aren't you missing the panel behind the right rear wheel?
For your version of the fuel tank, there is panel (5WA201429) but then you would need to change the fuel tank strap to the one with a bracket (5WA201656) and I can't see a screw installed for the other mounting point on your tank, behind the mounting point for the purple panel in your picture.
You can see it here, between the rear axle and the fuel tank. Ignore the red circle...
Also, the panel behind the rear axle in the picture exists in some different versions, mainly on the Octavia. Don't really know if it works when you have an extra muffler behind the rear axle like you have.