Sounds like you are married to a single concept, and exploring several solutions - which is cool.
I think you covered all the really simple solutions, anything else may be more complex. However the one solution left on the design table moves less mass/weight than any yet shown.
Hard to explain in words, but imagine a piano hinge at the lower bumper edge. The license plate taillight lower edge moves up. The bottom panel is a large "U" in section.
You have an unsealed with a gap/crack where the lower hinged moving "U" slides up into the upper shell (
also a "U" for strength), an open overlap joint.
Think of it as a hinged dustpan. The plate and lights would also have to be hinged, I've seen this done somewhere, at least the plate when the gas cap is behind it.
An upside down (
and backwards?) version of the Russian STOL jet below.
Tails Through Time: The Experimental STOL Demonstrator That Fooled NATO
In all of the designs I would allow air to escape out the tail, just in case the joint between the car and tail assembly is not 100% air-tight.
EDIT-1:
Like a dryer vent with triangular return sides.
https://www.pinterest.com/pin/380906080958536898/
EDIT-2:
I sketched it up, looks to be a drag penalty for being operable, open air-gaps not helping much. A cable and pulley system could operate it. Or just add some springs so it moves upon impact - that is if you don't mind a few scratches on your lower panel.