But does aoa remain constant during cruise at max range speed? Neglecting wind/temp changes etc.
A modern FMS attempts to do so, assuming 0 wind and calm air...
Regarding the first (wind), the airspeed for max range will have to increase in a headwind and decrease in a tailwind. The limiting speeds (or AoA) will be the speed at which the drag curve begins to rise sharply for the headwind, and max endurance speed for the tailwind.
Regarding the second (calm air), AoA will not truly remain constant in any turbulence, because different amounts of lift (and therefore AoA) are required at different load factors at a given speed. Therefore, it is better said that the average or nominal AoA will remain essentially constant.
If you set an FMS to fly at ECON Cruise at Cost Index = 0 (Max Range), the speed will decrease with decreasing weight, and adjust for winds. I assume there is also a temperature factor, but I don't know what the adjustment will be.
Also remember that we fly with the "artificial" constraint of constant altitude (or step climbs to discrete altitudes). Ideally an FMS would also adjust altitude to the current EXACT altitude for the weight and temperature, but in reality it adjusts for the current altitude and gives its best estimate of the optimum altitude. When that optimum altitude equals one to which we can be cleared, we climb/descend to it.