now all you need to do is time your tackle well enough so that your foot ends up under their leg and it will be a red card always?

The copenhagen player did not intentionally put his foot under rashford though. He was going for the ball and beat rashford to punch. It is no different than when fouls are given when a defender puts his foot between the ball and a player who trying to shoot at goal with the result of the player kicking the defender's foot instead of the ball. In that situation it is a clear foul so why not here. Also please stop giving malicious intentional plays as examples

the defender tries to go through him and in doing so his foot is slid under rashford's foot, this should never be a red.

The defender does not go through rashford, rather he tries to go to the ball before rashford and rashford is late to shield the ball. Rashford at no point was between the ball and his man from whom he was trying to shield and by the time he was, he had already stomped on him. You are given a yellow if you make a late challenge and red if the late tackle you make is dangerous. On the other hand if you time the tackle well, no matter how dangerous it would have been if the ball was not there, it is not a foul(unless you catch the player on the follow though studs up). Why is it that dangerous late tackles can be punished with red cards but a guy being late to plant his foot while shielding the ball and hence planting his foot on the guys ankle (pretty dangerous) cannot be punished similarly with a red¿?

ball shielding happens all the time

So does people lifting your feet high up to control high long balls. Many wingers and full backs do it. It is only a foul when you do it close to another player and only worthy of a red if you catch a player with your high foot. I expect the case here to be similar. Executed correctly, there is no problem with shielding the ball but you have to ensure that you reach the position from where you have to shield before your opponent.

this is nothing like that though

In both cases players took action which in a vacuum would be legal but because they did not account for the presence/positioning of their opponents, both of them are fouls. And because in both cases their foul ended up being something which can cause major damage to the opposition player, it ended up being red card worthy.