As above, the OP really needs to get to grips with travel & border crossings. Are they even sure they can fly into Europe then?
On the assumption that there are no border issues, the unfortunate fact of the matter is that others' previous experience with respect to accommodation is irrelevant this year.
First of all, all usual vacation patterns are up in the air - I'd be expecting far more people to be vacationing at home than abroad with a knock on effect on availability.
A lot of accommodation will possibly be operating at limited capacity, again reducing availability
Some options, I'd imagine would be totally off limits such as hostels, again reducing availability.
The Danube is a great ride but it could potentially kill off your girlfriend's interest in ever doing anything similar again.
Finally, have you contemplated the prospect of a resurgence of the virus, a localised lockdown and the difficulty of dealing with all that in a foreign language?
It's a great idea....but for another time.