You can get better through the literature itself. Improving your technique doesn't have to be a separate activity. In fact, for most situations I think it's better that it NOT be separate. My personal exceptions are a little octave work and a little scale work, and that's it. No "drills" or etudes, except for etudes I actually want to play as literature (practically none). Can you give an example of a specific aspect of playing that you'd like to improve, and also a specific piece or two that you'd like to play? Maybe we could put some ideas together.
I sorted this problem out a while ago with logic. Work on literature that you can handle and enjoy but devote a percentage of time to the less enjoyable asspects of your playing/i.e. technique.The better you get, the more you forget how difficult piano was when you first began and perhaps even get a little lazy on pushing your technique once you have a comfort zone. Once upon a time even you must have found trills hard etc. You must be resolute
rc: You can find ongoing large arpeggios in a lot of music. Faure and Chopin used it all the time, Medtner slow pieces, some Brahms, Field, Barber, some of the Mendelssohn Songs w/o Words, some Schubert. Well, Liszt, but you're looking for intermediate repertoire. There is a wealth of lovely material on which to practice this particular skill, and if you choose someone who changes key a lot-- say, Faure or Chopin-- you'll get to practice big chords in practically every key and chord. You can also take one of the classic patterns from the literature and just try it in every key yourself. That's quite fun. I'm always arguing against (much) technical practice that's isolated from actual pieces, but maybe I did more of that in early years than I thought. It was a game. I always wondered if I could figure out how to do things in different keys or rhythms just for fun, and I suppose this did serve as some technical practice too! So maybe I don't know what I'm talking about. I hate that. But I think you could have a really good time working on the big-chord thing in any of the above. Good luck!