> Naturally, this is a fundamental source of inefficiency.
It's also not correct. The stabilizer in the back (or more generally: the wing in the back) needs to have a smaller angle of attack than the wing in front. In the common case (wing in back much smaller than wing in front) the wing in the back often is designed with negative angle of attack to create enough margin, but it's not strictly speaking necessary.
There are planes (like the Lilium you mention, or the planes designed by Burt Rutan) that have a small wing in the front (a canard) and a large wing in the back, providing most of the lift. In that case the wing in the back obviously needs to have a positive angle of attack to create the lift for the plane to stay in the air.
That may look like a special case, but it is not, aerodynamically speaking. The rule is the same for all types of planes: for stability, the back needs a lower angle of attack than the front, and that does not necessarily mean it needs to be negative. It also means the plane's center of gravity does not necessarily have to be in front of the front wing.
See https://www.av8n.com/how/htm/aoastab.html#sec-basic-stabilit..., and more specifically https://www.av8n.com/how/htm/aoastab.html#sec-canard-same.
I don't really know whether canard configurations solve the inefficiency problem of horizontal stabilizers with negative angle of attack: canards necessarily have a high angle of attack, which also creates drag.
(Note that all of the above is only relevant for planes with classic static stability (which is almost all of them); planes with relaxed static stability and fly-by-wire are kept stable by computer instead of aerodynamics.)