Interesting- what you are saying is the opposite of what is more common. You say that the AC is not cold while driving at freeway/highway speeds.

It's usually the exact opposite with a failed aux fan. While around town or slow traffic, the condenser is not flowing enough air through it (because the aux fan is not working). Typically the AC works fine at highway speed because, well, you're moving at speed and air is flowing through both the radiator and AC condenser.

The expansion tank is a weak point on the car, the expansion tank and aux fan both fail around the same mileage, along with a myriad of other parts.

If you end up doing the aux fan, the front bumper will have to be removed. Here's a video DIY I posted up last year when we did our fan. Funny enough, I replaced the expansion tank shortly after as it had a hairline crack.

E53 front bumper removal:

https://youtu.be/QkPxUSSa0L8

E53 aux fan removal and replacement:

https://www.youtube.com/watch?v=N_K0KPHsteA