I am a 33 year old female from the US, I did this tour by myself in April 2018 and I LOVED it.
I opted for the regular bike instead of the e-bike, and I was happy I did, as the route is mostly flat and the regular bike is much lighter than the e-bike. My bike seat was padded, and with padded bike shorts as well I was never uncomfortable. The bags I was provided had plenty of space for changes of clothes, hat, water, cameras, everything (bring lots of sunscreen!).
My luggage was always at the hotel ahead of me, even when I thought I was early to arrive. The days did get long towards the end of the trip, it's a long way! But I got stronger as I went, and ended up with some calves of steel. Everyone I met along the way was super nice, most people wanted to chat with me and were very friendly.
I never felt unsafe. The local bike tour company was always just a text or phone call away. They checked in with me throughout the week, I had zero issues whatsoever! There were plenty of people on the path, even in the shoulder season. In fact, I recommend going in the shoulder season because it is less crowded, I imagine it gets super busy in summer.
Definitely stop by the monastery in Engelhartzell where the monks brew their own beer, do the hike above the river bend at the Donauschlinge hotel in Schlögen (it's short and worth it), get to Melk early so you can enjoy the beautiful village, and make lots of stops of food and drinks! On that note - I opted not to get the dinner package, and I usually ended up having dinner at the hotel anyway because it was convenient, but I enjoyed the freedom of getting to choose.
I am new to cycling, but it is a fantastic way to travel and see a new place. These self-guided tours are perfect opportunities for solo cyclists (especially solo females) to get into bike touring. This is definitely just the beginning for me.
P.S. German Lesson 1: "Donauradweg" means Danube bike path, just follow the green signs :)