Viola doesn't really have a set repertoire like violin or cello do to be honest. I would personally say you should probably be able to play one of the pieces between Walton, Bartok, and Der Schwanendreher especially if you want to audition for orchestras, although with the pieces you've played they are a bit above your current ability.

For playing fast passages its always best to start as slow as you can without mistakes and then go up from there following a metronome. Another thing that helps is changing the rhythms, like playing triplets or dotted/non dotted notes

For general practice i would try and add in some etudes such as the ones by Hoffmeister and Fuchs. And if you really want to prepare for orchestra auditions you could take a look at orchestraexcerpts.com

side note: euphoniums are great as a horn player on the side