My birthday is in mid-April and I have often gone to the Wachau to see the apricot blossoms as a present to myself. Your dates might be slightly early, but this has been a warm winter, so you might get lucky for flowers. Regardless, this is a beautiful time to visit as the early, fresh green leaves will be sprouting on the trees and the sun will be warm. Here is a link to a webcam which will start to show "apricot flower watch" towards the end of March so that people can time their visit correctly for the blossoms - https://www.wachauermarille.at/wachauer-marille/webcam
On bicycling, I biked on my last visit with my parents who are in their late 70s, early 80s. It is entirely flat and easy. I recommend at some point crossing the river at the Weissenkirchen/St. Lorenz ferry and biking or walking through the orchards around Rossatz. You get great views of Duernstein across the river and the orchards are lovely.
I would also note that Grein, recommended above, is not in the Wachau.