You can use any other detergent. But the Miele detergent is great.
I don't use any other.

Not known to me. Try it out, but I personally advise against it.
The set dosage can be reduced to approx. 20% and the washing result is still very good.
Washing powder can be used without any problems.
There are always Miele promotions with 25-30% discounts and the multi-packs are much cheaper.

There are empty refillable containers, suitable for Miele washing machines, which can be filled with other detergents. These containers are also available from Digitec.