However, isn't endurance at a maximum when total drag is at a minimum?
No, for this you need a minimum power speed.
Your glider has a certain ammount of Gravitational Potential Energy.
To maximise the endurance you need to be losing that GPE at the lowest possible rate. Energy / Time = Power.
Since power = force x speed you need to fly the speed that gives the smallest product of Drag x TAS.
(You'll find this below minimum drag speed, its the point where a line from the origin to the Drag versus IAS curve is perpendicular to the tangent. Being at a lower density altitude helps since this will give the smalled possibe TAS associated with that condition.)
So in fact we are not aiming for Min drag as such, which may influence the rest of your observation.
But anyway... where does CL come in? Well, lift forms part of the total aerodynamic reaction, but from an
endurance point of view we are not concerned with efficient lift production. The more drag you add, the less relevant CL is. Add infinite drag, and no lift is required yet the object will stay up there all day.
pb