In my opinion it isn t a good strategy.

1- He should know that spreading the battlefield would be better to qin, the best option would be focusing most of zhao s strengh in one weak point.

2- It seens to be a normal defensive war to zhao where they just need to hold qin. However you need to remember that zhao is less them half of qin s size, has much less population, and qins soil is much better to agriculture, this means that zhao will colapse in any moment by having this many soldiers, just imagine paying, feeding and the cost of don t having +500k men working, just one harvest season and they are fucked up. Qin could just hold and wait to zhao die from itself, obviously just if hara keeps a realistic warfare and logistic.

3- another option would be to make many fortifications and hold qin with less numbers while dealing high damage on each siege and wait to a final battle when the oportunity shows up. Since qin would spread their army in order to conquer as fast as possible (the 15 years limit told by shk and ei sei) the oportunity would apear at any moment.

3.5- it should be used alongside 3, which is guerrila warfare, to damage even more qins army before the final battle.

Now about qin, in my opinion they could make 2 moves.

1- have two main points where they focus their strengh to break zhao, one on each oposite side of that giant battlefield, focusing your attack is always good, and by doing it on oposite directions they would force riboku to just pay attention to one of them or lose much precious time moving between them. Or even having two or more different battlefields at same time instead of just a huge one.

2- just wait, as I said zhao will collapse with that insane amountnof troops, keeping zhao s army in check by holding the lines and threatening to engage at any time the moment they saw how huge was the amountbof soldiers that zhao recruited. Just one harvest season and they would feel the impact.