Tipping in Germany and Austria is similar and more like US than some other European countries (like France where tipping isn't expected or as common), just not quite a smuch as in the US, so more like 5-10 pct in restaurants.

I don't know what you are asking about hotels or who you would tip, so will leave that one to others to answer. I don't stay in expensive hotels with concierges, porters, etc.

for taxis, again, about 10 pct would be the norm, I believe. I don't know what you are booking but it sounds like a taxi.

For your reserved transfer from the airport -
as with other taxis/ transfers 10 % is fine - even a little less still ok
This known as Trinkgeld - literal translation - drink money.

I had an associate, a local banker that never tipped any amount to a taxi driver. While I do not agree with this - just an example

For restaurants - check menu closely- many restaurants will include a service charge of a fixed amount on your bill.
Thus your tip need not be excessive.

I see North Americans seem to tip much more than area people. Not needed to over due this except if exceptional service is provided.