We followed the Danube route through Austria, which was very straightforward and well signposted. It requires crossing the river several times on small passenger boats, which were around 5 Euro for the 2 of us and the bikes. There is a well signposted detour about 20km out of Linz (going East) to Mauthausen concentration camp and museum. It is a long, tough climb, with a 14% gradient in places.
We found campsites on our route in Austria to be small and expensive, but with standards similar to Germany (very good.)
We were surprised that food and drink was cheaper in Austria than Germany. It was fairly standard to find large pizzas in a restaurant for 8-9 Euro.
The ride from Vienna to Bratislava is nicely paved and well signposted. Bratislava itself is small for a capital city, and easy to navigate. We criss-crossed a bit between Hungary and Slovakia until we turned fully South at Komarno. The Slovakian side has better surfaces and less traffic.
Blog posts for Austria & Slovakia: