Interesting project.
 
1) flushing the radiator is always an important step, looks like your pump got gunked up (second time at end of video?) - by residual left in the radiator or hoses?
 
2) My "ideas regarding bleeding the air out"


 A) Vacuum filling is one possibility to not mix the air with the coolant while filling, if you have the equipment


 B) Air wants to rise (travels up hill) in a liquid.  So when bleeding you need to hold / turn the assembly to assist the movement of trapped air


 C) Hold assembly with pump at lowest point, radiator with hoses at the bottom of radiator.  Now gently shake the unit. (Best to also do this just before & when installing AIO into your PC, to get air pocket into correct part of AIO - away from pump & the hose end of radiator)


 D) Get the air out of the pump, usually requires multiple start / stop cycles, to stop cavitation (air in pump)


 E) Leave it undisturbed for a few hours, micro bubbles should form larger air pockets (reduce any foaming) & then bleed again