This may not be the answer you want , but what I'd do is record the walzes vienniece off one of the cheap
keyboards you're talking about.
You could record audio and make your own loops. Or if the keyboard has midi out plug a cord into your interface ,
pick one of the keyboards in GB and go for it.
do you have the JamPack "Symphony Orchestra"? The re are nice "Classical Waltz Piano" loops.
P.S. Before I had the JamPacks, I simply used software instruments loops from a 4/4 measure as a starting point and edited them to 3/4 measure loops.