I'm no expert so maybe others can chime in, but I think without buying new equipment, you have a few options for software of varying costs and UI...anybody else--please feel free to correct me if I am wrong on any of these. The two that immediately came to mind are:
Roon
Pros: Awesome for building a library. Great iOS app with awesome interface. Should be able to manage all of your files with ease.
Seems to work with a QNAP NAS. Also will allow you to do multi-room audio easily with any sources that have airplay and Roon works with Sonos. Also easily pairs with Qobuz or Tidal if you want a high-res music subscription service. For me, Roon has blown everything else out of the water in terms of it's UI, ease of use and function.
Cons: It is expensive and a subscription fee, or you can buy the lifetime subscription but it's expensive. I believe Roon is limited to CD quality audio via Sonos since it would use airplay. Doesn't connect with Spotify or Apple Music.
JRiver
Pros: cheaper than Roon. I do not have a ton of experience with it so maybe someone else can chime in. I did give it a trial and thought it was not bad but Roon has much better features for what I wanted specifically.